Before to add a co-owner into your panel, he must create an account on Minecraft-Hosting.pro. Except if he has already an existing account.
1. Click on the "Manage access" to access at this page.
2. Add the email account of the player you want he sees your server, click on the "ADD" button.
3. You can see here the account is accepted. ( firstname.lastname@example.org ) If you put the wrong email, the system will don't show you the email.