If you are a team admin that is verifying the completion of a challenge, you need to:
1. Go to the messaging tab (bottom second from right)
2. Click on 'Team Challenges'
3. Click on the Challenge
4. Toggle to 'Results' at the top
5. Click the grey box beside the names of the users that have completed the challenge
Once they are marked complete, there will be a green check mark beside their name, as well as a completed date underneath (which is the date the challenge was marked as complete).