How to assign task to multiple users at same time?

I make task management system and i don't know well how to assign task to multiple users at same time?

This is my AssignmentController Store function.

public function store(Request $request)
{
    $request->validate(array(
        'assignment' => 'array|required',
        'assignment.*.assigned_to' => 'required',
        'task_title' => 'required',
        'task_description' => 'required',
        'done_at' => 'sometimes',
    ));

foreach($request->get('assignment') as $assignment)

    $assignment = Assignment::create(array(
        'task_title' => $request->input('task_title'),
        'task_description' => $request->input('task_description'),
        'assigned_to' => $assignment['assigned_to'],
        'done_at' => $request->input('done_at'),
    ));
}

This is my assignment.create page where is 3 field assigned_to, task_title and task_description done_at is optional

<form method="post" action="">
        @csrf
        <table>
                <tr>
                    <td>Staff Name : </td>
                    <td>
                        <select name="assignment[][assigned_to]" id="assigned_to" multiple>
                            <option value="">Select One</option>
                            @foreach ($staffs as $staff)

                                <option value=""></option>

                            @endforeach
                        </select>
                    </td>
                </tr>
                <tr>
                    <td>Task Title : </td>
                    <td><input type="text" name="task_title" class="form-control"></td>
                </tr>
                <tr>
                    <td>Task Description : </td>
                    <td><input type="text" name="task_description" class="form-control"></td>
                </tr>
                <tr>
                    <td>Done At :</td>
                    <td><input type="time" name="done_at" class="form-control"></td>
                </tr>
            <td><button class="btn btn-primary" name="submit" type="submit" value="submit" id="submit">Submit</button></td>
        </table>
    </form>

via Chebli Mohamed