A forum for your social group is optional. When you reach 25 members you can request a forum if you decide you want one. The social group itself doesn't change or go anywhere, but it gets a new link at the top of the page:

Visit the forum of this group.

The last time I checked, anyone could see (and read) the forums of Public social groups but only members could see the forums of the other kinds. To post in any social group or its forum, you need to be a member of that social group.

I think the above additions are correct.