ads on facebook. good content with interesting story. also, you should have at least 10 members already in your group to convince ppl your group is good. How to do that? it's easy create 10 accounts by your own.
tinycoin is correct, I'll go a step further and say this... You should also populate the 10 (or however many you make) accounts with many Friends, then invite them all to the Group. It's easier then you think. This is how virality works, except usually of it's own accord. Do it right and nobody will know the difference and your Group will be massive soon