Sometimes, invitations to join your team can time-out or become obsolete, if the coach sends another one. You can send a request to join the team to your coach, or ask them to share the Team Key with you.
Another reason for this may be that the team has reached its user limit. Ask you coach to check this (and remove team members, if needed).
If you are having problems joining your team, please send us an email to firstname.lastname@example.org with your information, team name and any error messages you are receiving.