code: public void grantMembership(Jid jid) throws XMPPErrorException, NoResponseException, NotConnectedException, InterruptedException {
          changeAffiliationByAdmin(jid, MUCAffiliation.member, null);
      }
docs: Grants membership to a user. Only administrators are able to grant membership. A user
  that becomes a room member will be able to enter a room of type Members-Only (i.e. a room
  that a user cannot enter without being on the member list).
  
  @param jid the XMPP user ID of the user to grant membership (e.g. "user@host.org").
  @throws XMPPErrorException if an error occurs granting membership to a user.
  @throws NoResponseException if there was no response from the server.
  @throws NotConnectedException
  @throws InterruptedException