Forum OpenACS Q&A: Response to How to assign users to groups

Also, group administrators (members of the group with role "administrator") will see an "Administration Page" link at
http://your.com/groups/site_wide/

(Where site_wide is the group's short_name. Everyone will have a site_wide group.)

To add someone to your group,

http://your.com/groups/admin/site_wide/member-add?role=member

If the group membership policy is open (or wait), users can add (or nominate) themselves.

The ACES branch of ACS Classic has some pages for listing groups and letting users suggest groups.