How to correctly share a folder? I would like to share a folder within a small lan (5 client mac).
I have a partition with a folder 'Office' inside. I would like to assign to all folders and files the same ownership ('UserOffice') and use the same username for all my colluges. 

I've shared the folder with the following permisions: 


UserOffice read/write
Administrator read/write
Everyone  read/write


The problem is that a user can connect as 'guest' making it so, the permission doesn't work.

Is it necessary to remove 'Everyone read/write', or am I missing something else?
You have just identified why directories were invented to allow one server to define for a group of computers what users belong to which groups so that when sharing files, the permissions match so that computer Y will see the restrictions hosted from computer Z properly.

The first step is to decide how and where you will enforce the mapping of users to groups so that you can then set up sharing in a way that will work for you.

You'll want to decide if unix permissions (number based users and groups) is workable or if you need real ACL. Nothing prevents you from setting up each computer to mirror the others, but this doesn't scale well as you add people and computers.