Hi tricon
Until now there is no possibility to manage users group memberships from jOpenSim backend. For me this was something totally up to the users to handle their memberships. But in your case this question definately makes sense ...
If you planned only ONE specific group, users should be added, it could be solved easy by adding some TRIGGERs (MySQL 5.0.2+ required for this) to the database tables of jOpenSim without the need of changing one single line of code in it. But if the user should be able to select from a list of groups, this would be totally different and would involve quite a lot of changes.
Assigning a user to a group from the backend would for sure be an easier way to solve this and I added this now to my todo list for jOpenSim
Greetz
FoTo50