How do I add an admin user to my account

I am not sure if understand you correctly. Does it mean that you want to share your forms with someone? 

If that's the case then you can add sub-users to your account and share the forms. The following guide will help you - http://www.jotform.com/help/232-How-to-Share-Forms-with-a-Sub-Account-User 

Or you can add get yourself added to someone else's account as a sub-user. This way you both can share the forms too but the other account becomes the main account.

Additionally, please also note that there is a limit to the number of Sub-Users you can add. It depends on the Pricing Plan you have for your main account.