For the uninitiated, Wave is a new Google service, kind of like a replacement for e-mail adapted for new browser technologies. "Waves" are kind of like a mesh between e-mail, IM, mediawiki, and a slew of other things, but it's limited to a preview edition. You gain nothing from inviting other people, and most actually want to be invited (other places have people still begging for invites). The invite scheme is similar to Gmail's a few years ago. Invites are still limited.