Item edit permissions for contributors

I can't find a way to allow contributors listed on an item to edit the item. It seems to only allow admins and the user who added the item to edit. I'm interested in having a many-to-many relationship between items and users.

You are correct, that is how the current how user permissions work. Contributors can make exhibits out of items in the exhibits but cannot edit anything other than their own materials.

At the moment, you can make individuals admin users and then they can edit any item they like.

Has there been discussion on how difficult it would be to add user edit permissions for specific items? Or should I take this up on the developer list?

From what I understand, it would be difficult and may not even be possible for individual items. Please, hop on the dev list and throw it out to the group.