It is very easy to add a co-administrator to your Widows Azure subscription. You just sign in to you account at http://manage.windowsazure.com and then follow these easy steps:
1. Navigate to Settings page
2. Click on Administrators in the top menu and then click on the Add icon at the bottom.
3. Enter the email adress of the co-administrator that you want to give administrative rights to your subscription. If you are account administrator for multiple subscriptions you might se several subscriptions in the list presented. Check the once that you need and click on the check mark at the bottom.
You are now done and the other person can sign in and manage your Windows Azure resources.