I did the Mega European tour this year (2012) between June and August and we had a fair few people in our group leave at Athens and we picked up a lot of Eastern European trip people. By this stage we had been together for roughly 20 days and everyone knew everyone and little groups do form but we welcomed all the new people coming onto the trip. You do have to realise though that people will already have made friends by the stage you join but the best thing to do is just strike up a conversation with people and you’ll be fine. Some of my best friends on the tour joined halfway and were doing the Eastern European trip.
A little advice would be if you are travelling alone and joining into a tour group that has already been together for a while and you are worried about being lonely or don’t feel confident going out into a city alone is when you have your free days (so not on the bus) just ask a group of people if you can tag along with them for the day and if they were like my bus group, everyone would be more than happy to hang out with you for the day. Just ask what they were planning on seeing for the day and if that is relatively the same thing then tag along.
I know it can be very hard but probably the worst thing you can do is to act shy and not talk to people. I was given this advice before going on my trip that you will 100% regret it if you don’t put yourself out there and get to know people early. Everyone is in the same boat as you and are probably just as nervous so the sooner you can start talking and making friends the easier and more fun the trip will be. Looking back I’m glad I tried to talk to everyone in the first couple of days and know a couple of people on my bus who took half the trip to come out of their shells but once they did they enjoyed everything so much more.
Sorry about the long winded reply but I hope this helps a bit. Let me know if you have any other questions.
Also I should note that I had the best time ever on my trip and can’t recommend Topdeck highly enough! Everything was great and I’m sure you’ll all have the time of your lives. I miss it too Katie! I can’t wait for when I can book my next trip.