Hi,
this issue is related to MDL-50526 and MDL-50524
I've discovered that the existent error message do not fit into the cell height:

This should be fixed so that the alert box fits into the cell.
Furthermore, it would be convenient, if the student gets an information box, analogous to the display of the teacher that explains, why it is a problem, that the user is member of more than one group.
Teacher's text-box:

The box could say something like that:
"The assignment requires submission in groups. You are a member of more than one group. To be able to submit you must be member of exactly one group so that your submission can be mapped correctly to your group. Please contact your teacher to get enrolled in a group."

Cheers,
Kathrin