Can I join after the start date?
Yes. Join with the invite link or group code as usual. See How to join a group. For a private group, the admin has to approve your request first.
Do check-ins from before I joined count?
You can backlog workouts you did after the challenge started by tapping the Date button on the new check-in screen. See How do I backlog a check-in? Some groups don't allow backlogging. See Check-in Verification Settings.
Can I catch up?
It depends on the group's scoring mode (see Scoring modes):
Days Active / Check-in count: you only earn points for days you check in, so the days you missed before joining don't count unless you backlog them.
Hustle points / Metrics: same rule. Your total grows from what you log.
Admins can also set a scoring limit, which may cap how much you can backlog in one day. See Scoring limits.
Clubs
Clubs rank members weekly, monthly and yearly, so you can join at any time and start competing from the next ranking period. See Group types: Challenges & Clubs.