Adds a user to the node-ref.
Adds a user to the node-ref. If it is already playing,
it successively calls user.add().
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?