So if a WG is wound up and then continues under another name with mostly the same people doing the same things the original WG did it's not really wound up, is it? It's just changing the sign over the door with business continuing as usual.
First, anyone can participate. The only cost is the value of your time.
Second, yes, there are the usual suspects -- the ones who've decided to spend a lot of their time on whatever the area of tech we're talking about.
Third, working groups have charters that delineate what RFCs they will publish. Sometimes the work runs out. Sometimes the people run out of energy. Sometimes the tech is 'done', at least for a while. Then the WGs shut down.
Fourth, sometimes new work gets brought to the IETF in an area where the relevant WG has concluded, so then a new WG _may_ get spun up to take on that work.
