To add a user with Admin role to your development team on http://itunesconnect.apple.com please, proceed as follows:
1. Go to http://itunesconnect.apple.com >> sign in with your Apple developer account login and password.
2. Navigate to "Users and roles".
3. Press on "+" in the top left corner of the page.
4. Fill in new "User information" as per below sample (firstname.lastname@example.org as invitee mail in our case) and press "Next".
4. Tick "Admin" in roles and press "Next".