From the ground up it's designed for social usage. I suppose you could use it in the manner you're wanting as you're able to disable Category/Group creation preventing users from making new Categories/Groups. As for file attachments GJ does come with a file integration allowing display and download of files on a tab within Groups. However, users are capable of leaving a group at any time if that's acceptable (it's certainly not an ACL system or anything of the sort); however, you can edit the template and remove the leave links if you wanted.

Didn't plan on it, but suppose can add a parameter to allow disabling of leave links on a global scale (affect all groups). Certainly won't be adding on a group by group basis.

forge.joomlapolis.com/issues/2928