To add a user with Admin role to your development team on https://developer.apple.com/ please, proceed as follows:
1. Go to https://developer.apple.com/ >> Account >> sign in with your login and password.
2. Navigate to "People".
2. Press on "Invite People".
3. Fill in the field "Invite as admin" with the invitee email (email@example.com in our case) and press "Invite".
Voila! The invite to join your development team on developer.apple.com with Admin role has been sent to firstname.lastname@example.org