An add-on for the Organic Groups (og) module that makes one group mandatory for all new users and/or requires new users to pick a group.
Choose one group that all new users will be put into. You can chose either an open or a closed group. Any new user will be auto-approved as member of the group. The group manager(s) of groups that a new user joins will get an automatic e-mail notice.
Requires the Organic Groups module.