Yo! Here's the difference:
Groups are for people of a common interest to discuss amongst themselves.
Forums are for anyone to chime in.
If you want to start a group, please make sure it has a relevant picture icon (music page for orchestrators and so on.) And please be sure the group is different enough from what already exists and is created for people of that common interest.
If its just to discuss a certain music topic that could be of interest to many or all composers, then make it a "discussion" in the forums.
In the interest of maintaining a valued COMPOSER community, I may delete Groups, Forums, Discussions, or even members at any time.