I can think of a few different ways to get the word out, depending on what you are comfortable with. One would be to go on-line and start a new group on Meetup.com. Another way would be to post a notice at your LBS or any other community bulletin boards. If you have a local cycling website, you could also put a posting there. You can describe your group to attract riders with similar interests: it is for enjoyment, not racing or training...
Many riders are intimidated by groups, so you might want to include what you anticipate your average speed to be and if you will be doing any pacelines or drafting.
We started a group at our church that has slowly grown by word of mouth over time to include friends and neighbors of our original riders, so if you can just get a few core riders, you will see it grow.
The only issue we have had is new riders sometimes don't follow the same safety rules the rest of us do, so any time we have a new rider, we start with a little rules of the road talk.
It is great to have someone to ride with. I know some people enjoy the solitude of riding alone, but I will only ride when I am with at least one other rider. We have built up to enough riders now that we all can usually find someone to ride with when we want to go for a ride.
Good luck!
Grits
2010 Trek 5.2 Madone WSD, SI Diva Gel Flow
2002 Terry Classic, Terry Liberator