Isn't the obvious way to give credit to contributors to ... uhm ... credit them in the game and in the description?
While Itch is a place where indie games can grow, it is not a code repository where there are multiple developers work together to create something. Itch is a store front. Buying a thing that has multiple "publishers" is kinda strange. So yeah, if there is an "official" way to list other accounts in your project as "contributors", it should be possible to add them in a way that does not give them all access.
But thinking about it, how is this handled now? If you have contributors and a game that can get revenue? Do they get any? Is it configureable?