Thanks for your reply, Melanie.
I did a test with a student who is in more than one group, one for the class group and one for the presentation group. When I tried to have this student do a grouped assignment, I got an error message saying that the student was in more than one group, so the assignment couldn't be done. When I removed the student from one of the groups, it was possible to do the assignment.
So students can be in more than one group, but how can we specify which group is to be used for an assignment? This might not be the same question as I asked before, but I feel like it's the same problem