I suspect more groups, like standards groups, would use it if everyone could just get an account immediately, not have to scrounge for an invitation. It would also make a lot of sense to more people if it was integrated with GMail (although not to me so much, I have GMail but don't particularly like how it works).
I get that Google's "invitation only" approach helps them scale up the volume gradually, but taking this approach to a new collaborative platform is very different to taking this approach for an e-mail client.
I use PGP Whole Disk Encryption, and I take it pretty seriously, so I can't upgrade to Snow Leopard until I have a version of PGP that works with it. There is a beta programme, which I haven't joined. Not clear if it's still running, it was support to finish a couple of months ago, in principle.