I'm assuming you may not want to broadcast your home address here on the public forums. If you are still up for it after the trail on Saturday, the easiest thing to do is invite everyone on the trail run with you.
The other thing you could do is make an announcement at the BBQ Friday night to let everyone know.
Heck if you want to get really committed to it, we could do up a little flier to put in the goody bags.
Usually a lot of people will meet up at a restaurant to eat dinner with the people from the trail run that day and then retire to hang at the campgrounds. So it could work to have the bonfire after dinner. What time that happens depends on how the trail runs go.