Unless they conspired the most they could get prosecuted for is traffic offense unless they actually took part in the assault of the family. By either participating in the assault or actively harassing and stopping the couple. If they did not take part in the actual intimidation they cannot be charged IMHO. Maybe USER can clarify.
Did they not conspire to go on a group ride the purpose of which was to violate traffic laws and intimidate other motorists? Everything that happened as a result of that group ride should be able to be charged against all riders.