I don't generally take any riders. If they ride in back, they can become a distraction to me, if they ride up front, they can distract my driver. I prefer a "no riders" policy.
I will allow a rider in the back under very, very, very limited circumstances. If I'm doing a hospice transport, I'll often let the closest family member ride with them. If I'm transporting a sick, special needs child, I'll usually allow a care-giver or parent ride.
Other then that, I don't need the distraction. Most of the people I encounter that want to ride along are, from the first, a total PITA. It's the tachylordosis relative, the demanding spouse involved in the domestic, or the drug buddy wanting to ride with their BFF that I encounter more often then anyone else.