To give someone admin access they need to have registered to take part in the challenge.
To give any registered user challenge admin access:
Log in to your admin website
Click on Users on the sidebar navigation
Search for the user by name / email (field to top right of table)
Click on either the user's name.
Open the "Edit" section on the user's details page.
Select "Admin (Can access admin area)" as the user's "Role", and then click the "Update User" button.
The user can then login via https://yourchallengeprefix.bigteamchallenge.com/admin with full permissions.