It's nothing complicated.
Start out with your buddies, chip in to get a website/forum secured (vb sites are common), and start recruiting people with Mustangs. You could have some cards made up with the groups name and website, and just leave them on Mustangs that you see.
Our local group started in 2004ish or so, with three guys... now there are probably 20-25 fully active members, but we aren't in a huge city (year round, anyway). It's a very close-knit group, a lot of us are great freinds and hang out outside of club meets.
Just stay away from club fees if possible, that runs a lot of people off.