This looks a great trip and one I'd like to do one day.  I'd suggest you get to Heathrow around three hours before your flight (make sure you go to the right terminal)!  When you get to departures, look at the big screen to find your check in area.  Hopefully there will be a friendly JY rep there to direct you and help with any questions you may have.  If you have been given some JY luggage labels, put one on your hand luggage as this will help others in the group to spot you.  Hopefully you will have made a few holiday friends before you even board the flight!  Have a great time!

There usually is an airport rep at Heathrow; however on some trips I haven't been able to find them!  If you're there around 2 1/2 to 3 hours before you fly, hopefully you'll bump into some fellow travellers at check-in; that's what happened to me on my last trip which was only last month.  I'm sure they won't mind you tagging along until you find your feet!  I usually stay at the Premier Inn (there are several around Heathrow); the beds are comfortable, the food is decent and they are reasonably priced.  Hope it all works out for you.