We have a car rack. doesn't have to be fancy but you want the bikes secure for highway driving. I ride around our neighborhood occasionally but it's only maybe 2 miles ...prefer longer rides.
Yakima and Thule sell car racks, at least in the west. ( there are others) we have a trunk rack. $100-150 you can get trailer hitch racks or rooftop racks depending on your vehicle. for 2 bikes, trunk racks are usually cheapest and easiest. you go to the yakima or thule web site, plug in your car model and it will match you with a rack that will fit.
rear spoilers on cars don't work with trunk racks. take the rack off and put it inside the car while riding or some jerk will steal the rack. or lock the rack to the car.
some local person has probably written a book on where to ride in your area. "riding the backroads around blankety blank" is a common title. usually says where to park, mileage, traffic, hills, if a loop is possible. we have books for here and where we travel .
we have a very close by trail, open to walking, biking and horses. I could bike to it but there's a high traffic section I prefer to avoid so I usually drive there. there are hundreds of miles of trails around here, some paved, some gravel. we do some trails but also roads, low traffic rural ones where we can create a loop. that's where a book helps, someone's done the groundwork. you can look over the book and decide if the choices are appropriate for what you want.
You need bike, rack, helmet, gloves, sunglasses, small tools, extra tube & pump to change a tire, maybe a map. water bottle & snacks if you go longer. I think padded underwear /padded shorts help a lot but not everyone agrees.
flared pants get caught in the chain. wear shorts or capris, tapered pants, or strap down the bottom flare.
start with mileage that's comfortable. Lots of people ride a couple miles and that's enough. my favorite is 25-35 miles..... but then there's Josie who can ride 100.
Dyngus Day? never heard of that. what's it for? 15 people is a lot of people at once. never did that before. my solution for that many would be pizza! I don't remember what you were serving but since I generally don't eat/cook meat, large groups are tricky.