agorakit/agorakit

use Illuminate\Database\Eloquent\Model;

class Group extends Model
{
    //
    public function users()
    {
        return $this->belongsToMany(\App\User::class, 'membership')->where('membership', '>=', \App\Membership::MEMBER)->withTimestamps()->withPivot('membership');
    }
    //
}

use App\Group;
use Illuminate\Http\Request;

class GroupMassMembershipController extends Controller
{
    public function create(Request $request, Group $group)
    {
        $this->authorize('manage-membership', $group);

        $members = $group->users;
        $notmembers = \App\User::whereNotIn('id', $members->pluck('id'))->orderBy('name')->pluck('name', 'id');

        return view('membership.add')
            ->with('group', $group)
            ->with('members', $members)
            ->with('notmembers', $notmembers)
            ->with('tab', 'users');
    }
    //
}