In iTunes Connect, click iTunes Connect in the top left corner of the window to get back to the main menu. Select Users and Roles, then click the Sandbox Testers tab. Click + next to the “Tester” title.
Fill out the information and click Save when you’re done. You can make up a first and last name for your test user, but the email address chosen must be a real email address as a verification will be sent to the address by Apple. Once you receive that email, be sure to click the link in it to verify your address. The email address you enter should also NOT already be associated with an Apple ID account. Hint: if you have a gmail account, you can simply use an address alias instead of having to create a brand new account