Location, location, location...one of our knitting groups folded after a few month, the other is still going strong. I think there were several factors involved, but location was the biggest.
One was held in a well-lit cafe (there are extra lights that get turned on when the knitters are there) with two comfy couches and plenty of room to pull up additional chairs (different kinds of seating is a plus). The cafe has a good menu with lots of choices (food and beverages, sandwiches, salads, coffee, wine, beer, etc). Music, but not so loud that it makes conversation difficult.