After over 20 years of networking in many formats, both formal and informal, I think it comes down to this. We prefer to do business with people that we know like and trust and so the role of any networking group is to facilitate the building of those relationships. To do this there must be chat time, there must be the opportunity for members old and new to introduce themselves to the group and there should be time set aside for proper 1 to 1 meetings so that people can really get to know each other.
Networking is about business but it is also about sharing information and giving support so there should be a mechanism (e.g. a website) whereby members can easily stay in touch between physical meetings. Of course it is good to know lots of people but I am not sure that large groups are the answer. I prefer to be a part of a number of smaller groups with a geographical spread where I can really get to know people and have a choice of people to whom I can refer business.
Above all there should be the minimum of formality but a genuine pleasure in being together and in seeking opportunities for each other. That’s it.