Hi Julia, I agree with Rose - don't hesitate, life's too short, go for it!!
My advice is - at the beginning of the holiday (i.e. over the first day or two) don't be afraid to approach different people and strike up conversation - sit with different people at mealtimes, try to chat to everyone in the group. I've made so many great friends through my JY holidays, people I wouldn't have met otherwise. You won't gel with everyone (just like in everyday life) but if you don't make the effort to socialise you will be missing out! Conversations often revolve around how many JY tours have you done, which was your favourite, where are you going next etc.....
At this stage, I've learnt that a lot of people tend to "switch off" a bit on holidays and may not be very proactive, so if lunch/dinner isn't included, I often approach a couple of people and ask them if they'd like to meet in the bar at a certain time and head out to dinner/eat together in the hotel and sometimes make a general announcement along the lines of "a few of us are meeting up for a drink/heading to XYZ restaurant for lunch/dinner at ...o'clock if anyone would like to join us." Also don't be afraid to ask the tour manager (when the meal isn't included) for some suggestions of where to eat and if they know if some of the group have made arrangements - by and large people don't mind you tagging along - some of my best evenings on JY tours have happened that way!! (I hope the others I was with felt the same....)
Cheers,
Bob