P_Key membership bit

What is the intention of the language

        1. P_Key
            A "Full Membership" P_Key (high-order bit is set to 1) MUST
            be used so that all members may communicate with one
            another.

in ietf-ipoib-ip-over-infiniband-09.txt?  Let's say that a node has a
P_Key table programmed with P_Key 0x0001 (ie the full membership bit
is _not_ set).  Is the intention that the node should not be able to
join an IPoIB link with that P_Key, or does the node just have to make
sure it joins the broadcast group corresponding to P_Key 0x8001 (with
the full membership bit set)?

Thanks,
  Roland