public function isMember(User $user)
	{
		if(!$this->isPersisted)
		{
			throw new LogicException('You can only check if a user is a member of a group that exist in the database.');
		}

		if(!$user->isPersisted())
		{
			throw new LogicException('You can only check if a user that exist in the database is a member of a group.');
		}

		return $this->users()->where($user->getPrimaryKey(), '=', $user->getPrimaryKeyValue())->count() > 0;
	}