Assuming that you have all the waypoints and the route already in Basecamp then just put them all into a single ‘List’ in your library (the tree structure on the left hand side in Basecamp). Then click on your list in the library so it is selected. Then from the menus select File/Export and your list name should be already there. That should be it all into one file.
I’m not sure how your friends can import it without first reading it back into their own copy of basecamp though. The file is a gpx so they may be able to just drop it into their GPS’s