Add support for "Chinese walls" to groups. Chinese walls are a way to restrict communications between sets of users. This is a complimentary feature to shared groups. Options should be: 1) Only allow group members to contact other members in the same group. 2) Only allow group members to contact members of specific other groups. 3) Prevent group from talking with members of specific other groups, but allow communication to all others (opposite of #2). All packets between the users should be restricted including presence, IQ, and messages. Users must also be prevented from talking to one another in group chat rooms. I'm not sure how group chat restrictions should be implemented, though...