Same thing happened up here on a casual sunday ride about 4 or 5 months ago. Double line of riders (all competent) riding in the shoulder/bike lane. Car from 2 lanes over swerves over to us and passenger leans out the window, yells and screams and KER-THUNK - not so much pats - but whacks one of us on the back as they sped past. Bikes and people went in every direction and it was a miracle nobody went down.
Quick call by one rider "GET THE REGO". It was got. So was make and colour of the car. This rider had a handprint welt on her body. 12 riders tromped into local police station to lay an assault charge. Police said they would handle it. Looked up rego on computer. Without giving details, it belonged to a person in a neighbouring suburb.
They gave them a friendly call. Driver denied it. No officer, it wasn't me. I wasn't anywhere in the area. Our rider had 11 other witnesses (us) and the driver kept on denying it. Police said it was basically our word against his and without independent witnesses, they couldn't do anything. The rider in our group pushed and pushed and when it came to the crunch, nothing happened.