For some reason I can't edit my question, but I forgot to add my public function index of my controller

public function index()
	{
		setModuleSession("usersPermissions","");
		$usersList = \User::all();
		$group_options = \Groups::lists('name', 'id');
		return \View::make('admin.usersPermissions.index', compact('usersList'))->with('group_options', $group_options);
	}

                $ignore =   array('_token');

                foreach ($_POST as $key => $value) 
                {
                    if(!in_array($key, $ignore ))
                        $user->{$key} = $value;
                }

You said that you have a group_id in your input. All the input is taken and the '_token' removed. But you should also remove the group_id from the input.

                $ignore =   array('_token', 'group_id');

Eloquent doesn't know what column does your table have. It will try to insert all the properties you give to a model.