I do not think that there are actual organizations to get people together to live in trees, at least I did not find any sources that confirms this.

It seams like a good Idea. even though these trees are known to live 2000+ years, they do grow and can be hazardous to your house, but in all fact some people actually do live in trees. http://www.43things.com/entries/image/137083?t=e a picture of a tree house.
http://www.livescience.com/strangenews/060907_ap_redwood_tallest.html a little info on red wood trees.

http://en.wikipedia.org/wiki/Julia_Butterfly_Hill a story of a girl who lived in a redwood tree for 738 day to keep loggers from cutting it down.

do you realize, every time the tree huggers go into a forest to protest, it costs that county thousands to go clean up their trash? what a joke they are.

There are much more effective ways to get trees from getting cut down. Most tree huggers just do it for attention and to make it look like they really care more than others and they at times are effective. But it is just one form of protest, protest being one way to foster change. If you want to go live in a tree go for it, I did it when I was a kid. If you want to go be a tree hugging hippy that it good too. But just remember that hippies can be pretty ignorant and annoying too. Be a scientist. It’s much cooler.