Hello, I'm citing my answer from this forum post: https://itch.io/post/567237
1. Enter the project Dashboard
2. Click More/Admins
3. Enter the username and click Add admin
4. Send the agree to link to collaborator
5. Turn on the "Contributor" toggle
Let me know if that helps, anyone with more experience doing this please verify if that is correct.
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: https://itch.io/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.
