Paste the email alias you're looking for in there. A contact information summary box should appear; click on the "Show contact information" icon (highlighted below) to get more details:
Step 2: Find the MyID on the Contact information page
In the main Contact information summary view, the MyID you seek is in the center, under the Chat section (I've highlighted the region):
You can click on the icon to the right of the MyID to copy it to the Clipboard: Once copied, the icon changes to a green check:
Now you can use it to create accounts!
MyIDs are usually a combination of letters (usually initials) and numbers (yz09876, or abc1234), while email aliases are usually much longer.
A quick rule of thumb: If it's longer than nine characters, it's an email alias.
If it has a dot in it, it's an email alias.
Sometimes email aliases can be short; if you've got a list of Drupal accounts to create, you'll always need to look up the MyID, as sometimes people have short email aliases that differ from their MyID by trivial ways (I've seen one-character differences). Always check.
If you're using this in the Drupal Username (and CAS Username) fields, you'll need to remove "@uga.edu" from the end of the MyID.
To manage any part of your web site, you will have to be logged into the Drupal CMS.