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 ther

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.