Groupsites allow you to rename your subgroups to any name you choose (Chapters, Teams, Workgroups, etc.) up to 20 characters long. Of course, you can then also name each individual subgroup -- or Chapter, Team, Workgroup, etc. -- as you see fit.
You can rename your subgroups by going to MANAGER > Group Settings > Subgroup Controls.
From the "Subgroup Controls" page, make your desired change in the "Subgroup Settings" area. The "Advanced" link within this area will even allow you to change the "subgroup" name (or other, as you choose) for use in plural contextual references throughout your Groupsite.
When you rename your Subgroups, the system will automatically rename your SUBGROUPS tab reflect the new name you've chosen.
Note: to avoid confusion of terms throughout the Groupsite, one name you cannot pick is "groups." We don't allow the use of the word "group(s)" when renaming subgroups.
If you want to change the name of a particular, individual subgroup (say you made a spelling error when creating it, or simply want a new name for the subgroup/chapter/team):
You can find controls to change the name of a individual subgroup by logging-in to the relevant subgroup, and then clicking the "Admin" controls link (beneath MY SETTINGS) in the top right corner. Note, you can change the name of a particular subgroup but you cannot change its underlying URL. If you need to change both the name and URL of a subgroup, consider creating a new subgroup with the URL and name you desire and deleting the old subgroup.