SOLVED UPDATE: You need to create a new record rule with a specific domain. The domain can be something like this:
["|", ("user_ids","!=",False), ('user_id','=',user.id) ]
I've been watching and reading many tutorials about permissions but I can't figure out how to set a group (A) to see other group's documents, without letting the group (A) to see ALL the documents? I need the group A to ONLY see another group of users' documents, not all. Thank you!