The users of my system are able to join multiple groups. When the user is viewing a group and uses the "add " links from the group details block the post is restricted to just that group with the way I have things setup. That works great.

Now it is possible for a user to post content outside of a group, however when they do this they get a list of groups to post to. Unfortunately they can select multiple groups which I do not want. For this particular case, is there a way to force the user to either select no groups or only one group?