Brian Solis uncovered an interesting feature of Facebook groups – if you unsubscribe from a group, you cannot join it ever again. This is portrayed as a feature to improve the social dynamics of groups, by making people use care about which groups they invite others too, and which they expect.
Update: Actually, you cannot be re-invited to the group ever again. If it is an Open group, it’s possible that you can choose to rejoin it (if someone has tested this, please write in commen...