As the only formally distinct class of people found in all open source projects, committers deserve special attention here. Committers are an unavoidable concession to discrimination in a system which is otherwise as non-discriminatory as possible. But "discrimination" is not meant as a pejorative here. The function committers perform is utterly necessary, and I do not think a project could succeed without it. Quality control requires, well, control. There are always many people who feel competent to make changes to a program, and some smaller number who actually are. The project cannot rely on people's own judgement; it must impose standards and grant commit access only to those who meet them. On the other hand, having people who can commit changes directly working side-by-side with people who cannot sets up an obvious power dynamic. That dynamic must be managed so that it does not harm the project.
In Section 4.3.4 in Chapter 4, we already discussed the mechanics of considering new committers. Here we will look at the standards by which potential new committers should be judged, and how this process should be presented to the larger community.
In the Subversion project, we choose committers primarily on the Hippocratic Principle: first, do no harm. Our main criterion is not technical skill or even knowledge of the code, but merely that the committer show good judgement. Judgement can mean simply knowing what not to take on. A person might post only small patches, fixing fairly simple problems in the code; but if the patches apply cleanly, do not contain bugs, and are mostly in accord with the project's log message and coding conventions, and there are enough patches to show a clear pattern, then an existing committer will usually propose that person for commit access. If at least three people say yes, and no one objects, then the offer is made. True, we might have no evidence that the person is able to solve complex problems in all areas of the code base, but that does not matter: the person has made it clear that she is capable of at least judging her own abilities. Technical skills can be learned (and taught), but judgment, for the most part, cannot. Therefore, it is the one thing you want to make sure a person has before you give her commit access.
When a new committer proposal does provoke a discussion, it is usually not about technical ability, but rather about the person's behavior on the mailing lists or in IRC. Sometimes someone shows technical skill and an ability to work within the project's formal guidelines, yet is also consistently belligerent or uncooperative in public forums. That's a serious concern; if the person doesn't seem to shape up over time, even in response to hints, then we won't add her as a committer no matter how skilled she is. In a volunteer group, social skills, or the ability to "play well in the sandbox," are as important as raw technical ability. Because everything is under version control, the penalty for adding a committer you shouldn't have is not so much the problems it could cause in the code (review would spot those quickly anyway), but that it might eventually force the project to revoke the person's commit access— an action that is never pleasant and can sometimes be confrontational.
Many projects insist that the potential committer demonstrate a certain level of technical expertise and persistence, by submitting some number of non-trivial patches—that is, not only do these projects want to know that the person will do no harm, they want to know that she is likely to do good across the code base. This is fine, but be careful that it doesn't start to turn committership into a matter of membership in an exclusive club. The question to keep in everyone's mind should be "What will bring the best results for the code?" not "Will we devalue the social status associated with committership by admitting this person?" The point of commit access is not to reinforce people's self-worth, it's to allow good changes to enter the code with a minimum of fuss. If you have 100 committers, 10 of whom make large changes on a regular basis, and the other 90 of whom just fix typos and small bugs a few times a year, that's still better than having only the 10.
The first thing to be said about revoking commit access is: try not to be in that situation in the first place. Depending on whose access is being revoked, and why, the discussions around such an action can be very divisive. Even when not divisive, they will be a time-consuming distraction from productive work.
However, if you must do it, the discussion should be had privately among the same people who would be in a position to vote for granting that person whatever flavor of commit access they currently have. The person herself should not be included. This contradicts the usual injunction against secrecy, but in this case, it's necessary. First, no one would be able to speak freely otherwise. Second, if the motion fails, you don't necessarily want the person to know it was ever considered, because that could open up questions ("Who was on my side? Who was against me?") that lead to the worst sort of factionalism. In certain rare circumstances, the group may want someone to know that revocation of commit access is or was being considered, as a warning, but this openness should be a decision the group makes. No one should ever, on his own initiative, reveal information from a discussion and ballot that others assumed were secret.
Once someone's access is revoked, that fact is unavoidably public (see Section 8.4.5 later in this chapter), so try to be as tactful as you can in how it is presented to the outside world.
Some projects offer gradations of commit access. For example, there might be contributors whose commit access gives them free rein in the documentation, but who do not commit to the code itself. Common areas for partial commit access include documentation, translations, binding code to other programming languages, specification files for packaging (e.g., RedHat RPM spec files, etc.), and other places where a mistake will not result in a problem for the core project.
Since commit access is not only about committing, but about being part of an electorate (see Section 4.3.4 in Chapter 4), the question naturally arises: what can the partial committers vote on? There is no one right answer; it depends on what sorts of partial commit domains your project has. In Subversion we've kept things fairly simple: a partial committer can vote on matters confined exclusively to that committer's domain, and not on anything else. Importantly, we do have a mechanism for casting advisory votes (essentially, the committer writes "+0" or "+1 (non-binding)" instead of just "+1" on the ballot). There's no reason to silence people entirely just because their vote isn't formally binding.
Full committers can vote on anything, just as they can commit anywhere, and only full committers vote on adding new committers of any kind. In practice, though, the ability to add new partial committers is usually delegated: any full committer can "sponsor" a new partial committer, and partial committers in a domain can often essentially choose new committers for that same domain (this is especially helpful in making translation work run smoothly).
Your project may need a slightly different arrangement, depending on the nature of the work, but the same general principles apply to all projects. Each committer should be able to vote on matters that fall within the scope of her commit access, and not on matters outside that, and votes on procedural questions should default to the full committers, unless there's some reason (as decided by the full committers) to widen the electorate.
Regarding enforcement of partial commit access: it's often best not to have the version control system enforce partial commit domains, even if it can. See Section 22.214.171.124 in Chapter 3 for the reasons why.
Some projects automatically remove people's commit access if they go a certain amount of time (say, a year) without committing anything. I think this is usually unhelpful and even counterproductive, for two reasons.
First, it may tempt some people into committing acceptable but unnecessary changes, just to prevent their commit access from expiring. Second, it doesn't really serve any purpose. If the main criterion for granting commit access is good judgement, then why assume someone's judgement would deteriorate just because he's away from the project for a while? Even if he completely vanishes for years, not looking at the code or following development discussions, when he reappears he'll know how out of touch he is, and act accordingly. You trusted his judgement before, so why not trust it always? If high school diplomas do not expire, then commit access certainly shouldn't.
Sometimes a committer may ask to be removed, or to be explicitly marked as dormant in the list of committers (see the following section for more about that list). In these cases, the project should accede to the person's wishes, of course.
Although the discussions around adding any particular new committer must be confidential, the rules and procedures themselves need not be secret. In fact, it's best to publish them, so people realize that the committers are not some mysterious Star Chamber, closed off to mere mortals, but that anyone can join simply by posting good patches and knowing how to handle herself in the community. In the Subversion project, we put this information right in the developer guidelines document, since the people most likely to be interested in how commit access is granted are those thinking of contributing code to the project.
In addition to publishing the procedures, publish the actual list of committers. The traditional place for this is a file called MAINTAINERS or COMMITTERS in the top level of the project's source code tree. It should list all the full committers first, followed by the various partial commit domains and the members of each domain. Each person should be listed by name and email address, though the address can be encoded to prevent spam (see Section 126.96.36.199 in Chapter 3) if the person prefers that.
Since the distinction between full commit and partial commit access is obvious and well defined, it is proper for the list to make that distinction too. Beyond that, the list should not try to indicate the informal distinctions that inevitably arise in a project, such as who is particularly influential and how. It is a public record, not an acknowledgments file. List committers either in alphabetical order, or in the order in which they arrived.
 Note that commit access means something a bit different in decentralized version control systems, where anyone can set up a repository that is linked into the project, and give themselves commit access to that repository. Nevertheless, the concept of commit access still applies: "commit access" is shorthand for "the right to make changes to the code that will ship in the group's next release of the software." In centralized version control systems, this means having direct commit access; in decentralized ones, it means having one's changes pulled into the main distribution by default. It is the same idea either way; the mechanics by which it is realized are not terribly important.