Miss B, I have no experience riding on a group tour, but I've been touring, so here's my thoughts:
- earplugs. Always a good idea. You never know if you are sleeping next to a snorer's tent.
- bug repellent.
- carry a light for your helmet. It doubles as a night light in the tent.
- start early if you can. Try to get in as many miles as you can before it gets hot.
- drink before you are thirsty and eat before you are hungry. When I've toured, I've had a small breakfast at the campsite, and then 'second breakfast' about 10 miles or so into the ride.
- keep in mind this is not a race. There's no prize for finishing ahead of other riders.
- take photos. I took a thousand of them on my big tour last year. As Spinnaker on the Touring Forum once told me, "20 years from now you'll find yourself thinking of that small cafe you ducked into to get out of the rain, and you'll wish you had a picture of it."
- Oh, and most importantly, have fun!