These days when it comes to Web 2.0 beta launches, sites don't just launch their products and services to the masses anymore. Remember when Google Analytics beta came out? Maybe you don't, but I was one of the 'exclusive' to try it out a long time before it became available to the masses.
In fact I've been lucky enough to be in early on a lot of betas. It makes sense too, these sites and products create extra marketing buzz for themselves, while also getting their products and services effectively tested. By the time they officially launch, thousands, if not millions, of people want their products. Much better than just, wham, here it is.. use it.
Quite often when invite only betas are going around, invitees are given a few extra invite tokens that they can give out and that's where InviteShare comes in. InviteShare has built an exchange for them.
Not all the exclusive web 2.0 sites are in there, but there are a lot and this service has been growing at a frantic rate (2-4k Alexa over the past week).
All in all to summarize, InviteShare allows people to give and get invites to closed sites. Will it be the next web 2.0 hit to follow Myspace, Digg, YouTube and others? Maybe not that big, but time will tell.