Adds a user to the node-ref.
Adds a user to the node-ref. If it is already playing,
it successively calls user.add().
Retrieves the main group of the Proc.
Retrieves the main group of the Proc. If this group has not been assigned yet, this method will create a new group.
Retrieves the main group of the Proc, or returns None if a group has not yet been assigned.
Removes a user from the node-ref.
Removes a user from the node-ref. Note: If the node-ref
is already playing, it currently does not call user.remove(),
but this must be done by the caller.
XXX TODO -- perhaps we should change that?