Select audience for discussion posts or student (as opposed to global) bulletin board

It would be great if users had the ability to set an audience in the discussion section. For example, I would like to create a discussion with myself and 2 other students and the instructor, but not have the rest of the class able to see it. This would be extremely helpful in facilitating collaboration for group assignments, especially if we can attach multiple files, or put links to files in our file tabs (which no one at our school can seem to figure out who can see them and when, or how to notify someone there is a file we'd like them to see). Also, it would be great if we could put private messages (viewable by administrators for obvious reasons) on another student's bulletin board, so we could send messages saying something like, "I'm going to miss class today, can you share your notes?" or "I forgot to write down some thing [whatever it was] that the teacher said." or "Hey, we're having a study group tonight in the dorm, can you come?" etc.