bbPress.org

There’s an excellent Private Forum plugin:

https://bbpress.org/plugins/topic/hidden-forums/

And the ability to create and define custom user groups to access a particular forum:

https://bbpress.org/forums/topic/custom-user-groups

You mentioned a few social networking type plugins… have you checked out buddypress, the social network extension to WordPressMU?

https://buddypress.org/about/

It might be overkill for what you’re looking for, but at least some of the feature/plugins do exactly the sorta things you mention (galleries, pm, etc.).

Good luck!