Users of your challenge can register easily by going to the landing page that's automatically created for your challenge.
This usually looks something like https://DEMO.bigteamchallenge.com, where "DEMO" would be replaced by the unique domain name you chose when creating the challenge.
Once on this page they can use the simple 'Sign Up' form on the right hand side of the page.
Within a couple of minutes they should receive an activation email, which they will need to follow before the account is made active.
Then they should be up and running (or walking!).