Full access shared folder for two unprivileged users

I am an unpriviledged user A in the same group G as another unpriviledged user B. We want to share a directory. I want to create a shared folder, be it /tmp/shared. How do I set its rights? Seems I want to set 2770. But what does B prevent from creating a folder with 700 in there? Can I force full access to all files any user creates in my (!) folder I shared with them? And, of course, also full access for them. Does this even work with this rights model? If not, how do I do this with ACLs? As far as I understood, ACLs help me to prevent that B *accidentally* creates a file without granting me access, but does not prevent if B does it *intentionally*.