At Strapi, the admin user is someone who has access to the Strapi Admin Panel. If you want to add an admin user to your Strapi account, you can easily do it from the Customer Portal by clicking here.
1 - Access your account with your email address. You'll receive an email with a password to access your account.
2 - Select the project you want to add admin users to
3 - Edit your subscription
4 - Add admin users to your account and update your subscription
If you're having any trouble with any of the previous steps, please contact us at support@strapi.io mentioning "Add users".