Mine never did, but I was warned they would. They preferred to walk on the trailer I keep parked next to the car. Maybe because it's taller? But no, mine didn't ever get on my car.
Things to be prepared for?
1 - NOISE
2 - Guineas needing help with fences ... They will run back and forth screaming to get through field fencing that their bodies would fit through. Or to be let back through a gate they JUST flew over and don't know how to fly BACK over.
3 - Noise
4 - flying birds that you can't stop by clipping their wings. Someone (I think on this forum) once said they were convinced guineas could fly bald. I'm inclined to think they were right.
5 - Did I mention NOISE?!?!?!
I like my guineas though. They were EXCELLENT bug control in the yard and garden. They do scratch the ground so make sure they can't damage young plants. They also escape regularly and go annoy the neighbor so I have to herd them home before he loses patience. You can't keep them fenced in unless you put netting overhead. They WILL fly out. Mine have alerted me once to cats stalking my chickens, so they are proving a little more useful. Mine are also EXCELLENT foragers. They barely touch any feed so are economical to raise. Today I ate my first guinea ... She was cast out of the group and I tried to add her to the chicken flock but she wanted to rejoin the guineas. After some miserable months I finally realized she would be an outcast and miserable no matter what ... So I had guinea curry tonight. It was very good, and if I could get them to raise keets it could prove a VERY economical source of almost-chicken, since mine won't eat commercial feed.
Oh, and if you don't get them cooped in time at night, they WILL sleep in the trees. Mine have cooped themselves maybe 3 times. They usually have to be herded in. But I've never lost one to any predator, and they roam far and wide and have spent more than one night out of the coop.