Warn users if they are about to leave a group with members

[fasib]

There should be a warning if you are already a member of a group with multiple members and click on Create group or Accept another invitation

[fasib]

Or the options (invitations & create group) could be removed and a notice added that you can only join one group and leave it.

[leMaik]

I'd choose the first approach. If you don't see your invitations, you may not want to leave your group as this could lead to you being on your own then.
I guess this is somethink that @LittleHelicase should decide.

[LittleHelicase]

I would also prefer the first approach, a simple warning that informs the user of the consequences. Creating a new group copy the current state of the sharejs document? But accepting an invitation would definitely result in loosing the current status of ones solution.