A group membership can be added without filling in the group

One-sentence description

A group membership can be added without filling in the group.

Current behaviour / Reproducing the bug

1. Create a group membership for a user but don't fill in a group even though it is bold
2. breakage

Expected behaviour

1. Create a group membership for a user but don't fill in a group even though it is bold
2. a error message that is more helpfull