There are also people that haven't played it before (or any RTS by the sound of your friends) and might really enjoy it.
The trick is to give up on trying to get the first group to play and to increase the second group when you have the chance. I have a good ~15 friends who play starcraft ranging from noob (or UMS) to competitive. I think that's a lot and it's entirely from "Do you like this? Great, I'll let you know when we're playing" or "Oh well, we'll play Left 4 Dead then".