blue-twilight/app/Group.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Group extends Model
{
    /**
     * The attributes that are mass assignable.
     *
     * @var array
     */
    protected $fillable = [
        'name'
    ];

    public function users()
    {
        return $this->belongsToMany(User::class, 'user_groups');
    }
}