Auto Group is an extension for Jomsocial which will automatically join members to groups based on their selection from a profile field which you as the administrator setup.
It also has the ability to send PM suggestions for groups to join based on profile selections.
For example say you have an online classroom that teaches math, spelling and history.
You can add a custom field with jomsocial that will ask what they would like to learn, math, spelling or history. When the user selects math, they are automatically joined to all groups with the word math in the title or from the category math or from the groups with math in the title from the category called math.
Whenever the member updates his or her profile and changes his category he is added to the groups from his current category.
The member or group admin are responsible for leaving the groups they have joined.
You can also use Multi-Select Elements so that if your user chooses math and history, they will be added to both groups and to take it even a step further you can have multiple multiple select elements ie a group of check boxes for category of groups a multi-select drop-down as well as a radio element, enabling you to handle large amounts of group selections.
We have also built in a logic system allowing you to join to groups if a profile field equals, doesn't equal, is greater than or less than a value of your choice. This can be used to join members to groups based on their age or the level of education which they indicate with a profile selection.
The possibilities are endless with the Auto Group Joiner for Joomla and Jomsocial.