Well, google makes it pretty easy to collaborate, and the view/zoom feature along with the terrain maps I find exceptional, so here's how I plan a route and load it into my gps:
Google maps using the get directions feature I create something like this:
And share the google link to all parties until we all agree on a route, then I take it and at the bottom of the left hand column, save it and "Create new map".
When I go to my maps, it's available as a download as a KML file, then I use
www.gpsvisualizer.com to convert it to a gpx, download and import it into Mapsource to do final tweaking and then share out the GDB file to all the participants. Every city or destination you used to shape the route will become a waypoint in the end product, so use them strategically to alter your route rather than "dragging" the blue line, that will create an awesome track, but then you'll need to really tweak the route to make something out of it in Mapsource.
Routing you have to do manually, but the tracks are excellent and need minor tweaks, and if you ride offroad, then you'll love the tracks you get from this method. I'd happily follow a blue line of my proposed route, then to trust the auto-routing adventure seeking adlibbing tendency of my Garmin.
