1. Click on Hosted Organizations option from your control panel.
2. Click on Org name.
3. Click on Mailboxes.
4. Click on the Mailbox of the user for which you need to add Aliases.
5. Click on E-mail Addresses Tab.
6. Enter the E-mail address and click on Add E-mail address.

For Example: You want to add an Alias demoalias@fordemo.com; then in the E-mail address field enter demoalias and click on Add e-mail Address.
You will notice that the new alias has been added.

TIP:   You can set the Alias as Primary by selecting it and clicking on Set As Primary