If you have “entered” and “confirmed” (paid the entry fee for a event) you can “join” that trading event at any time during the event.
If you have not “entered” and “confirmed” (paid the entry fee), you cannot join the event once it has started.
It is your responsibility to join and pay for each trading event before it begins. We generally recommend joining a few minutes prior to the start of any event.
Next suggested article: When can I compete in a TradeOff™ event?