The Chat section of the app (Press "More" on the navigation bar at the bottom of the screen to access this section) contains chat groups and messages:
Organization chat (if your organization has engaged GoJoe) - this is for health and fitness GoJoe engagement with your colleagues all year round, whether in or outside of challenges
Direct messages - will appear in this area.
...and when you create or join a team in a challenge, you'll also automatically be added to:-
Team chat - this contains all of your team-mates,
Challenge chat - this has all of the participants. This can be also found within your challenge.
...and If you are part of a journey, you'll be automatically added to:-
Journey chat - this includes all journey participants