Hmm... what resolution do you use exactly for the game? Maybe the debug log contains some information about what was going on. To enable logging, please go to your game directory (rightclick on rw in steam -> properties -> local files -> browse local files) and open the config.properties file with a text editor. Now set "game_debug_console" to true, save the changes, and run the game again. Once you're in main menu, quit the game, then go to the "Logs" subfolder in your game dir. Please upload the latest log file here
Thanks for the log! It looks like the resolution is causing this issue It works fine on full hd (1920x1080), but smaller resolutions seem to cause trouble with the friend list (if the game is already started with a smaller resolution, it doesn't work to switch to another resolution - it's necessary to restart the game instead)... I'll put that on my "things-that-need-to-be-fixed" list
In the meantime, the regular steam friend list should do the trick (you can either join people there, or people can invite you).