Cornell U. Startup Wiggio Passes 100k Users

They got to 100k users because the person creating a group can add however many email addresses they want to each group. There is no invitation step, like there is with Google or Yahoo groups.

So they can sign up a thousand people a day with a hundred people creating groups of 10.

And since it's free text & email blasting, it seems like this would be a spammer's dream come true.