Earlier quoted context omitted.
...there is an "email me when a wave gets updated" option, you know. They even let you decide how often you want to get emailed. I chose once/day, but you could go all the way up to one email for every blip updated/added. And this way works with all email, not just gmail! There's a link in that email to send you straight to what's updated. How much integration do you want, exactly?
Did they launch with these features? I distinctly recall thinking "how do I know who's on this thing?" and repeatedly going back to the site in hope that someone else I knew finally used the site and sent me a message. The technology might be great but the launch and execution were really, really weird.
Well, they did originally have "who's online" functionality, but they had to yank it when someone found an exploit, and it took them forever to get it working again.
The "email me" option was much later - around the time they went from "Preview" to "Beta", I think. And as thorax said in his reply to me, Google flubbed the UI/usability of the preferences.
As far as "who has a Wave account", it's the same scheme as email. You get a Wave address; you're responsible for letting all your friends know what your address is (e.g., mine's in my HN profile). Probably could've been improved by hooking into Facebook/Twitter/etc. and spamming out your Wave address to all your friends...