public boolean isUser(JID user) {
        // Make sure that we are always checking bare JIDs
        if (user != null && user.getResource() != null) {
            user = user.asBareJID();
        }
        return user != null && (members.contains(user) || administrators.contains(user));
    }
Returns true if the provided JID belongs to a user that is part of the group.

@param user the JID address of the user to check.
@return true if the specified user is a group user.
xmppserver/src/main/java/org/jivesoftware/openfire/group/Group.java