If you have a local hippy/alternative/wholefoods/vegan/fairtrade shop you might be able to advertise in there - that would be a good place to find other EC families in my experience! (Our green/hippy type shops often have adverts for AP groups, Waldorf kindergartens, Goddess camps, etc - always too far away for us to get to though.) Do you have any local friends you were planning to start off with, a sort of instant group?
I suggest meeting in your home to begin with, or perhaps on a park since it’s the right time of year for it! Hiring somewhere just adds cost and complication when you don’t yet know how many people you’ll get.