Can an employer who is paying mileage to go to an offsite workplace force you to take pengers and carpool?

Can they make you ride with someone else if it not a company vehicle? I need to know where the information comes from if they cannot. I need to be able to cite where it came from. It's very important. The driver that is currently being forced to take pengers is not insured and is being required to transport propane as well tools and equipment required for work. This is not a construction job. Most of us are women and uncomfortable taking one or two men in the vehicle. (we are not ist, just cautious about ual harrasment). The employer is making it sound like if we don't carpool we won't have jobs, but no one thinks that just one person should be paid to get to work. We all need the extra funds. Any information would help. We are in Michigan. Any state or local laws relevant would also help. Thanks!!