Yeah, either you downgrade to version 2007, or your two friends upgrade to version 2008. Or, to follow Microsoft's suggested work-around, you could install Streets and Trips 2007 (along with your existing S&T 2008), and then do the trip planning in S&T 2007. Share the S&T 2007 files with friends, and then open the S&T 2007 file in your S&T 2008 when you are ready to drive.
Terry, it has been a wish of many users that this be addressed, but it is still the way it is .