How to add Alias ID for User

Refer below given steps or Watch Youtube Video Tutorial

1.     Sign in to the Google Admin console.
2.     Find the user account. You can either search or browse:
3.         Search: Enter the user name in the search box at the top of the Google Admin console, then click Search accounts.
4.         Browse: Click Users.  If you created an organizational structure, select an organization from the organizational structure on the left.
5.     Select the user and click Profile, which will display the user's profile.
6.     Click Add an alias.  
7.     Enter the user name in the Add an alias text box. The user name will be the portion of the user's email address that appears before the @ sign.  
8.     Select the domain for the alias. If you have secondary domains associated with your Google Apps account, the field to the right of the @ sign is a drop-down list with the available domains. The domain you select will be the portion of the user's email address that appears after the @ sign. (If you have a domain alias for your primary domain, every email address in the primary domain automatically has an email alias address in the alias domain.)  
9.     Click Save changes. It can take up to 24 hours for the alias to become available.

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