Save array in BD with laravel 5.5

I have a create for the registration of users where there is a MULTIPLE SELECT to assign several entities to the user.

And he throws an array for each option I frame.

So I have the multiple select:

<label for="entity_id">Asignar Entes</label>
  <select id="ms" name="entity_id[]" multiple="multiple">
     @foreach($entities as $e)  
       <option value="{{$e->id}}">{{$e->name}}</option>
     @endforeach
  </select>

And the driver to save the user's data:

public function store(Request $request)
{
    $user= new \App\User();
    $user->name=$request->name;
    $user->email=$request->email;
    $user->entity_id=$request->entity_id;
    $user->role_id=$request->role_id;
    $user->password=bcrypt($request->password);

    if($user->save()){

     return redirect()->route('users.index', $user->id)
         ->with('info', 'Usuario guardado con éxito');
    }
    else{
        return redirect()->route('users.index')
         ->with('info', 'No se guardó el usuario');
    }
}

And the fields of the database, where entity_id is the field that I want to save:

How do I store that array in my database? thanks in advance.