You need the team’s invite code, which an administrator creates. If you are not in any team yet, after signing in tap “Join a team”; if you are already in one, go to Settings → “Join with a code”. Type the code in “Invite code” and tap “Join the team”.
If the staff has already prepared your card, the app asks “Are you one of them?”: choose your name and tap “That’s me”. You are in straight away, but until the staff confirms that it is you, you don’t appear in the roster, can’t be called up and get no ratings.