Each account created in Google Apps has a main e-mail address that is used to log in. That address you use to log in will be the one that appears in Drive, Calendar and other Google services. In addition, that account may have other addresses added as aliases which can be used to send and receive messages.
If you want your main e-mail address and the address that is displayed in your contacts in Google services to be .EUS, the .EUS domain needs to have been added to your Google Apps account. The article How do I add a .EUS domain to Google Apps? explains how to do this.
Once the .EUS domain name has been added, you will be able to change the Google Apps users. There are two ways of doing this depending on the number of user accounts that have been created in Google Apps.
If you have fewer than 50 users
If you have fewer than 50 users in Google Apps, the main address of the users has to be changed one by one. To do this, when you are on the administration console, select the “Users” section.
The user accounts you have in Google Apps will be displayed. To change the main address of a user, click on the pencil icon.
A new window will open showing the user’s main information (name and surname(s) and e-mail address). You will be able to change the main address in the “Primary email address” section. Select the domain name you want to use for the main address. When you have finished, click on the “Rename users” button.
This will change the address and from then onwards you will log in with the address you have just changed. What was the main address will be added as an alias so that it can be used to send and receive e-mail. Most Google services will automatically be updated to the new main address, but there are some exceptions. https://support.google.com/a/answer/2622985?hl=en.
This change will have to be repeated with the remaining Google Apps accounts.
If you have more than 50 users
If there are more than 50 users in your Google Apps, the change of main address can be done automatically without having to change them one by one. Google Apps Script service in Google can be used to do this. Write to the Google support service and the steps that need to be followed will be explained to you.