In the bottom left corner of your screen, click on "Hosted Services, Storage Accounts & CDN"
Click on "User Management" on the top left of the Azure Portal.
Click on the "Add New Co-Admin" button on the top left of the Azure portal.
Enter the Windows Live ID (email address) of the person you wish to add as a Co-Administrator,
and click on the checkbox of the subscription you wish to add the "co-admin" to.