Adding a user in Office 365 is a straightforward process. Let's walk through the steps starting at Home | Active users | Add a user.
In the Add a user dialog that opens, at a minimum you'll need to enter first/last name, display name, and login name. You may also choose the user's domain, which will be the default for your tenant. As shown here, helpful tips will appear to the right on each field that you click:
Add user dialog
As you can see, contact info is optional as well as the default options for password generation, but you will need to specify a license for the user (or otherwise indicate that you want to create ...
