What tapped-out proposed is a good way to move one time the files, but if what you want is to gain access, you can use this:
Say your users are Foo and Bar, you are Foo and you want access to Bar's file:
login as Bar, go in finder and right click the folder you want access to. Click Get Info. At the bottom, you should see this:
You can use the plus(+) at the bottom left to add Foo to the permission of the folder. When you do so, give him Read & Write permission and there you go. You should be able to go back as Foo and fully access this precise folder in the system and do whatever you need with it.