Pay attention to what's going on around you. That doesn't mean you have to act like a quivering mess. It's trail riding. It's supposed to be fun. Just be aware of your surroundings, and of the other riders and horses.
If the person in front of you points out something like a large exposed root, a hole in the ground, a low branch that you'll need to duck under, etc. then pass the message along to the person behind you.
It sounds like this will be a large group and that may also mean you'll have a mix of riders and horses from all experience levels. Unless you know everyone there, don't assume they're all experienced but don't assume they're all novices either - and just because someone may claim to be "experienced" it doesn't mean they're necessarily good riders.
Don't crowd the horse in front of you - even if the other horse isn't wearing a red ribbon in its tail, that doesn't mean it won't kick. You are responsible for keeping a safe distance between your horse and the horse in front of you. A normally patient horse might still kick out if another horse insists on breathing on its "cheeks" the entire time.
If you must pass a horse in front of you, ask them ahead of time if it would be okay to pass and give them wide berth. Don't dawdle about it but don't blow past them either. Just move along and get where you're trying to go.
When climbing uphill (especially a large hill), don't come to a sudden stop or slow down as soon as you reach the top. Even if your horse is "blowing" a bit, continue moving on or find a clear spot off to the side for your horse to catch his breath so that the horses behind you can keep coming up.
If you stop at a creek or stream to water the horses and there isn't room for them to all drink at once, don't hog space. Once your horse has had a drink, move off to the side so someone else can move in.
Also, some horses like to "play" in the water, pawing at it with their front feet. I have a friend whose horse does this, and while it's cute and all. . .it also stirs up the bottom and splashes anyone riding or standing nearby. Sometimes it's also an indication that your horse is thinking about laying down for a little soak, so it's best to stop the behavior as soon as it starts and get out of the water.
Enjoy your ride!