Hmm, doesn't seem to work this way. I end up with duplicate entries. I had another idea using the diff($collection) function but I always get the following error:

Argument 1 passed to Illuminate\Support\Collection::__construct() must be of the type array, object given, called in ***/vendor/laravel/framework/src/Illuminate/Foundation/ProviderRepository.php on line 157 and defined

See an example here: http://www.neontsunami.com/post/new-collection-methods-in-lara...

I added the following two lines to my providers array in app.php

'Illuminate\Support\Collection',
'Illuminate\Database\Eloquent\Collection',

This is my code in User.php:

public function scopeNotProject($query, $project_id)
	    {	$all_users = Users::all();
	    	$participants = Project::find($project_id)->users;
	    	$nonparticipants = $all_users->diff($participants);
	    	return $nonparticipants;
	    }

and my View:

@foreach(User::notProject($project_id)->get() as $nonparticipant)
			          <option value="{{ $nonparticipant->id }}">{{ $nonparticipant->firstname . " " . $nonparticipant->lastname }}</option>
			@endforeach

As far as I can see, the passed item should be a COllection and not an array(?). I'm new to laravel so ANY help is appreciated! Thank you.