The two sites are not in any way connected.
This forum is privately owned by a gentleman named Rick Hall. It is open to the public at his pleasure.
The COG site is owned by and run by the organization Concours Owner's Group, which is a dues- paying, membership based organization. That said, their (our I guess- I am a member of COG) is open to the public at no charge, in that case at the pleasure of COG.
As both sites are privately owned and operated, either / both could be closed, closed to some members, etc., at any time. While open to the public at the moment, that could change (not that I suspect it will) at anytime- which is why I mentioned that they are both open at the owner's pleasure.
And yes, a lot of people are members of and participate in, both forums. Some people cross- post the same subject on both forums but certainly not always.
Like all institutions and organizations, each one has a 'personality'. From the forum point of view, there is not all that much difference but in practice, COG also uses their forum to notify people of events, rides, rallies, etc. that they put on.
Brian
I am fairly new here and have pretty much stuck exclusively to this site. I notice many members here also have membership on the Concours Owners Group forum. What is the main difference between the two? Am I missing something by not reviewing both sites.