

So, the most obvious thing is to run the command gksudo, which ought to prompt for the password on login. Your user enters their password and it all works.
Don’t do that.
Number one it’s bad practice to blindly enter your password and number two having a user editable script that you run blindly every time you log in is a real vulnerability.
If you can do it with groups, do that. Have everyone be in the group and have the permissions of the directory let them do what they need to do if they’re in the group.
If that doesn’t work for some reason (you can change steams file creation mask) then acls are the next thing to try.
After spending a few minutes reading about how steam handles .acf files and how unpredictable that is, the most elegant solution would be more storage.