Many people are members of several functional groups. Having access to files based on a single group designation is inconvenient. This often requires the user to switch between groups, using the newgrp(1) command, simply to gain the correct access permissions.
Supplementary groups makes it possible for a user to have access to all files at the group level, even when the groups differ. An example illustrates this problem:
Account erin is a member of group projectx.
Account scott is a member of group projectq.
Erin and Scott are working on similar programs within each of their own projects (projects X and Q), and they are in dispute. Laura, their supervisor, wants to compare the files to see how much they differ.
The difficulty ...