We've had numerous occasions where instructors have created Group Submission assignments and then learned that they can't edit the groups once the assignment goes live. Unfortunately, there is no indication in the GUI that this is functioning as intended.
At the recommendation of Ben Leveillee at URI, we're going to use the Message Bundle Manager to change the text when creating a Group Submission assignment to read as follows:
Group Submission - One submission per group
(PLEASE NOTE: This feature is not recommended if you would like to make future edits to this assignment or groups in this site. Selecting this option significantly limits the ability to make group edits, including adding or removing students from groups.)
I think that a similar wording should be considered for trunk, so that all instructors know that selecting a Group Submission assignment can limit their ability to edit the groups related to that assignment.