Laravel - Querybuilder with join and concat

Logan's answer got my started in the right direction. I also had to remove all of the 'user.' prefixes since it was calling the Users model already I suppose. This query worked:

User::select(DB::raw('CONCAT(last_name, ", ", first_name) AS full_name'), 'id')
                        ->join('users_groups', 'id', '=', 'users_groups.user_id')
                        ->where('activated', '=', '1')
                        ->where('users_groups.group_id', '=', $group)
                        ->orderBy('last_name')
                        ->lists('full_name', 'id');

Thanks everyone! Hopefully if someone else runs into this they will find guidance with this question.

You do not need to select in DB::raw(), See the example

User::select(
          'id',
          DB::raw('CONCAT(first_name," ",last_name) as full_name')

        )
       ->orderBy('full_name')
       ->lists('full_name', 'id');