Title says it all. There should be some kind of limit thought to stop advertising/spamming. The invited user should see it somewhere (for example in his profile page).
We've already covered something similar to this recently. With the friends/followers setup, you'll be able to post a status update that your friends or followers (depending on how you set up your privacy settings) to leave feedback, essentially inviting only your friends.
We won't build out a forum where you can invite/dis-invite certain users. There's too much overhead involved.
