These are some of the ways you can accommodate a latecomer:

1. Pass all of the racers through inspection in the Registration tab, then each racer will have a spot reserved for them. If they miss a heat, then you will have to decide if you want to run them separately and then manually enter their time into the heat they missed (only works for times scoring).
2. Adding a Bye racer or two. You have the same issue as above with them coming after a heat is already run.
3. Run them as a separate group, but then you'd need to manually tabulate the standings to factor them in (only works for times scoring).
4. Use the Manual Scheduling feature (Pro Version) to add in heats for that racer (only works for times scoring, since the racer may only be competing against no one else).
5. Restart the race, but of course, this is likely to be the most disruptive to the other racers.