Multiple races are displayed on the in-game home screen. Under each of them is a small information ribbon indicating if the race has already started, or how much time is left before the start. If the race is marked as "ongoing", it has already started. It is time to join it!
It is never too late to join a race, even is you missed the beginning! On Virtual Regatta, you can join at any moment. You will be added to the fleet, amongst the other competitors.