Make sure that they have a cell phone number for the tour guide in case there is a problem. I recently had a huge blowout with a car service at LGA because the driver was unable to find me (although I was waiting exactly where I was told to - and have spent a LOT of time flying in and out of LGA) and all messages were being relayed through the car service dispatcher - I was unable to speak to the driver directly. As soon as they finally agreed to give the driver my cell phone number so he could call me directly, we found each other immediately.