Show progress on large reference group additions

When deletions take a long time, the UI will display a message...

    The delete operation is still being processed...

We've noticed when adding large reference groups to composite groups via the UI, the browser will spin for exactly 3 minutes, and then return to the UI having made no obvious changes.  If you wait an "appropriate" amount of time and then check the UI, the change will have been made. 

Please consider adding a message similar to the deletion message.  As it is now, the user doesn't know that their change wasn't just eaten and may try it repeatedly.