Check the Just Cruising area, that's where people post about cruises.
Personally I just usually go to the lighthouse at North-West. There are always some people parked at the parking lot next to the light house and usually some of them are ready to cruise.
If you like slow / realistic cruises, I'm ready to do those almost always when I'm in TDU (my id is "Osmokv" and "Azoxid"). The speed that I like to cruise is traffic speed or a bit faster, and I stopg at red lights and stop signs. When I'm in a cruise, we usually pass cars only at safe straights where you can see that it's safe and no risk of crashing.
If you've never been on realistic cruise, you should try it. I find it much more enjoyable than those full speed "crazy" cruises. :) Of course, having a fast car available, sometimes we want to push down the gas pedal on a nice place, but even then the speed is controlled so that we can avoid crashing. ;)