When large numbers of participants have all registered to race at the same time, the field is split. In short, you don't have to do anything. Which race you'll be assigned to is automatically determined by a number of factors, including your iRating, Internet Latency, and Friends. The greatest weight is given to iRatings, so as to create close racing between similarly skilled drivers.

When the field is split, you won't know who else you'll be racing with until the countdown timer gets to zero and you press the JOIN button at the top right of the Racing panel. That will automatically take you to the correct race, where you can see who is in your session by clicking the ENTRIES tab once the simulation has loaded.