I will be traveling with a tour group soon. They have the ticket reservations and I will not get my ticket until the morning we travel. Can I add my Known Traveler Number at check-in?
Are you trying to get Pre-Check clearance? If so, it's best to add your Known Traveler number to the itinerary as early as you can, since the airline sends the PNRs to the TSA about three days before the flight. Of course, the airline can resubmit the day of the flight, but it's more work, and not all check-in agents know how to do it or want to take the time to do so.
You should be able to get the PNR from the tour company in advance. You can also obtain the PNR if you know the flight number, at the airline web site or by calling the airline. Once you have the PNR, you can usually edit the Secure Passenger Information to add the Known Traveler number using the "manage my booking" or similar function at the airline's web site or on the phone. Note that only airlines that participate in the TSA Pre-Check will even have the capability for you to enter it and to send it to the TSA for Pre-Check clearance.