The OpenEdge Hive

"Does one have to have a project to have a forum or can one just add a
forum on OE Hive?"

A Project is not required for creating a Forum.

All the forums on oehive presently are Organic Group forums. Anybody can create an OG. It looks like I can also create a standalone Forum, but I don't know what that would mean with regard to notifications and content etc, because all the other content on oehve belongs to a specific forum.

I think it's supposed to be possible to add multiple OG Forums for one OG, but I haven't seen in the menus how to do that.