public boolean isUser(String username) {
        if (username != null) {
            return isUser(XMPPServer.getInstance().createJID(username, null, true));
        }
        else {
            return false;
        }
    }
Returns true if the provided username belongs to a user of the group.

@param username the username to check.
@return true if the provided username belongs to a user of the group.
xmppserver/src/main/java/org/jivesoftware/openfire/group/Group.java