code: public function isMemberOf($id)
  	{
  		// ID not numeric
  		if (!is_numeric($id))
  		{
  			return false;
  		}
  
  		$groups = StringUtil::deserialize($this->groups);
  
  		// No groups assigned
  		if (empty($groups) || !\is_array($groups))
  		{
  			return false;
  		}
  
  		// Group ID found
  		if (\in_array($id, $groups))
  		{
  			return true;
  		}
  
  		return false;
  	}
docs: Return true if the user is member of a particular group
  
  @param integer $id The group ID
  
  @return boolean True if the user is a member of the group