I joined the Connecticut Plein Air Painters Society last year. There are about 200 members in this organization. Paint outs located in Connectiuct are planned during the spring, summer and fall. They have a member show every year which is very impressive. You need not be a resident of Connecticut to be a member or make a "pilgrimage" to CT to satisfy membership.
Here's their website for those who are interested.
Personally, I think it would be wonderful if there were regional plein air societies throughout the USA as well as national and international groups.
Smaller groups could then affiliate with the regional groups and the regional groups with the national groups similar to the pastel societies.
Hey the more crazy outdoor painters there are, the merrier. Who knows there may more of us than we think!!