modUser Join Group

//create the user
$user = $modx->newObject('modUser', array ('username'=>$ipn['info']['email']));
$userProfile = $modx->newObject('modUserProfile');
//join the usergroup and assign role
$user->joinGroup(5,4);

//set the fields
foreach ($ipn['info'] as $key => $ui) {
     $userProfile->set($key,$ui);
}
//add extended info
$userProfile->set('extended',$ipn['extended']);
$success = $user->addOne($userProfile);

//set the password as their phone number
$user->set('password',md5($ipn['info']['phone']));

if ($success) {
    $user->save();
    $modx->log(modX::LOG_LEVEL_ERROR,'Profile Created: '.print_r($_POST,true));
} else {
    $modx->log(modX::LOG_LEVEL_ERROR,'Profile Denied: '.print_r($_POST,true));
}

//set the password as their phone number
$user->set('password',md5($ipn['info']['phone']));

$user->joinGroup(intval($group), intval($role)); // where $role = '0' (= NONE in 'modx-combo-role' ExtJS element)

or 

$user->joinGroup(intval($group));

    /**
     * Join a User Group, and optionally assign a Role.
     *
     * @access public
     * @param mixed $groupId Either the name or ID of the User Group to join.
     * @param mixed $roleId Optional. Either the name or ID of the Role to
     * assign to for the group.
     * @return boolean True if successful.
     */
    public function joinGroup($groupId,$roleId = null) {
        $joined = false;

        $groupPk = is_string($groupId) ? array('name' => $groupId) : $groupId;
        /** @var modUserGroup $userGroup */
        $userGroup = $this->xpdo->getObject('modUserGroup',$groupPk);
        if (empty($userGroup)) {
            $this->xpdo->log(xPDO::LOG_LEVEL_ERROR,'User Group not found with key: '.$groupId);
            return $joined;
        }

        /** @var modUserGroupRole $role */
        if (!empty($roleId)) {
            $rolePk = is_string($roleId) ? array('name' => $roleId) : $roleId;
            $role = $this->xpdo->getObject('modUserGroupRole',$rolePk);
            if (empty($role)) {
                $this->xpdo->log(xPDO::LOG_LEVEL_ERROR,'Role not found with key: '.$role);
                return $joined;
            }
        }

        /** @var modUserGroupMember $member */
        $member = $this->xpdo->getObject('modUserGroupMember',array(
            'member' => $this->get('id'),
            'user_group' => $userGroup->get('id'),
        ));
        if (empty($member)) {
            $rank = count($this->getMany('UserGroupMembers'));
            $member = $this->xpdo->newObject('modUserGroupMember');
            $member->set('member',$this->get('id'));
            $member->set('user_group',$userGroup->get('id'));
            $member->set('rank', $rank);
            if (!empty($role)) {
                $member->set('role',$role->get('id'));
            }
            $joined = $member->save();
            if (!$joined) {
                $this->xpdo->log(xPDO::LOG_LEVEL_ERROR,'An unknown error occurred preventing adding the User to the User Group.');
            } else {
                unset($_SESSION["modx.user.{$this->get('id')}.userGroupNames"]);
            }
        } else {
            $joined = true;
        }
        return $joined;
    }

$role = null;

if (!empty($role)) {
   $member->set('role',$role->get('id'));
}

if (!empty($role)) {
   $member->set('role',$role->get('id'));
}

if ($roleId === '0' || $roleId === 0) {  
   $member->set('role', 0);
}