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 "Edit" beside the user
Tick "User can access admin area" on the next page and then click Update User.
The user can then login via https://yourchallengeprefix.bigteamchallenge.com/admin with full permissions.