Contacts many_to_many relationship with Users - how to use?

If you navigate to Admin -> Studio -> Contacts -> Relationships, you will see that Contacts has a many-to-many relationship with Users. 

I cannot access a contact's users via the API. I've tried `Contacts/:id/link/users`, `Contacts/:id/collection/users`, `Contacts/:id/link/contacts_users`, and `Contacts/:id/link/contacts_users`. How exactly can we access this relationship via the API?

Context: we are using SugarCRM as an API for a custom frontend, and we would like to leverage the many-to-many relationship between contacts and users to set up restricted visibility for assigned users. I'm aware that SugarCRM has a default assigned_user field, but we need to assign more than one user to contacts.