Does free help need legal contracts?

My game is almost complete, and it's almost ready for the final play test, proof read, and localisation.

For all the above, I have had offers from a small community of followers that have offered to do these things for free. If the game does well (or not), are they going to be knocking on my door asking for their cut in the profits?

Do I need to give these people a legal paper stating that they are only doing this out of the kindness of their hearts and won't be getting anything more out of it other than what I offered them; which is a copy of the game and name in the credits?