Symfony 3 send multiple forms

So I'm creating a web site like 'todo' list(something like todoist, trello). Now I'm trying to make that any user could asign tasks for other users. I've already made that any user could asign task for any other USER. But I would like to make that any user could asign the same task for as many users as he wish. And I'm a little bit stuck here. Not sure how to make that the user could select as many users as he wish.

Here is an example of my web page:

F.e. I would like to asign a task for users "admin" and "jonukas" in one form. Is it possible?

Any help is appreciated.

---I'm not asking to write a code for me I just need some advices or examples---

P.S. Sorry for my english. I hope you all understand what I'm trying to say.