From my understanding this does indeed give them full admin permissions to edit files. I'd recommend either keeping a backup of the project somewhere else or if you'd prefer total safety, they can add the game to a collection and display it on their profile.

This goes into detail about collections: /updates/game-collections-and-updated-profile-pages

Again though, I've never really worked with these things myself, so if someone else can verify that there's no way to add contributors without giving them admin permissions that'd be great.