When I setup up a dedicated server race fo me and my brother to race on. Firstly I run my dedicated server file in the rfactor main folder and select the cars, tracks and settings etc. Secondly I then start up rfactor and when it loads I change the settings for online game searching to LAN instead of internet search. Your game that you setup will appear and you enter it as usual.
I would tick matchmake annouce so your friend can find the game.
Hope that helps you and dont confuse the issue. If you want a few more players to join send me the details, if not no problems.