Dealing with Dependencies in Active Record Models — Option #1: Using a facade/service locator/global access

Invitation.php

<?php

class Invitation extends Model
{
    public function user()
    {
        return $this->belongsTo(User::class, 'invited_by');
    }

    public function send()
    {
        Mail::send('emails.invitation', ['invited_by' => $this->user], function ($m) {
            $m->to($this->recipient)->subject("You've been invited to my app!");
        });
    }
}

InvitationsController.php

<?php

class InvitationsController extends Controller
{
    public function store()
    {
        $invitation = Invitation::create([
            'recipient' => request('recipient'),
            'invited_by' => auth()->user()->id,
        ]);

        $invitation->send();

        return response('', 204);
    }
}