I have a friend who works as a guide in Tokyo and I think she's quite good. Nothing official, she just tours people on weekends (she has a regular weekday job). However, usually she uses her own car which means a group of 4 people max. It's quite tough to fit 6 people and a guide in a single vehicle unless you rent a minivan. Two vehicles would be a problem, I think.

There are self-drive companies that rent minivans. I don't know if my guide friend would drive a rental minivan but I think there is a good chance. Or maybe one of your party would be the driver.