A nice feature would be that you can start single player (vs bots) race, and it would end as soon as someone connects (or you could manually end the race immediately when you see someone connects), or they could connect on-the-fly (at least just for that round that started as singleplayer round, or this could be enabled in the settings or something). This would remove the effect of "having to sit in the lobby alone potentially 2-30 minutes before anyone joins and maybe if you alt-tab you don't even notice that someone has connected" etc.
Also I recommend adding friends as you play, that way your friends (who like to play GRIP) can see when you start playing and join you, or the other way around, or you can ask. (for instance, you can add me, I play 1-7 times a week usually roughly 18.00 - 21.00 GMT (6-9 PM) and usually there are people playing around that time.
In the Rollcage group people can create events when they want to play RC, why not expand this to GRIP too? Maybe that will become a thing a little later on (though hopefully there won't be a need for that:P) The game is still in development so hopefully MP will become more popular a little later on!
On a slightly different topic, good players can try to NOT to totally crush new players before they get properly in the game (maybe vote catchup on, dont fire at them constantly, be encouraging, give tips etc.), I bought the game for my friend and when I asked him "Tired of getting your ass handed to you?" when he quit, he just responed "Yes" and started to play another game xD) This is a thing a (for the time being) small community/playerbase must take consideration so that new players won't be scared away rightaway (just my opinion!)