Hi loralai
jOpenSim's database is the only database, where memberships in groups are stored, so yes, this issue can probably be solved only in jOpenSim (and/or in it's C# module). The database structure is taken from the good old opensimProfile module and might be a bit confusing at the beginning:
The membership of a user in a group is stored in #__opensim_groupmembership, that's correct. But the membership in the group roles are stored in #__opensim_grouprolemembership and what role is activated for a user by default is stored in #__opensim_groupactive.
There are quite some changes/bugfixes for these functions in the upcoming version of jOpenSim, but for example the group land functions will still not be implemented due to known issues inside OpenSim.
I will try to publish the new version as soon as possible. Just at the moment some stuff keeps me a bit busy, so I have to ask for a little patience
Greetz
FoTo50