GW 2012 setting up internal only email

We want to setup internal only email accounts by adding a new Class of Service and placing Internal and External groups in that COS. We created the new COS but are unable to add any groups. With that said, the Class of Service must be added on the GWIA and not the email server.
1) Why is COS only found on GWIA and not on the main email server? We will be required to log into the GWIA server to do email account maintenance (add new accts, inactivate, delete accts.)
2) Why can't we add groups in the Class of Service on the GWIA?
Thanks