Create a new Apple ID (eg: jamf.gsx@example.com) to use as the JAMF GSX user
After creating the account and verifying the e-mail address, go to http://gsx.apple.com and log in with the newly created Apple ID (ex: jamf.gsx@example.com)
When prompted, enter the Sold-to account number of your SSA/GSX and a Job function (you can fill this with any title you wish)
Accept the terms and click submit
Now log into the GSX as your GSX admin, not the Apple ID that you created in step 1
Click on "Access Requests" under your admin name in the left-hand panel
Select the access request for the account in step 1 and click approve
From the left-hand panel, click the disclosure triangle by "People" and select the Users folder
Select the Apple ID you created in step 1
From the right-hand panel, assign read-only access to the various categories
Click "Save" to save the permission changes