On their FAQ’s page it says:
Wait, but why does the authorization message say you need access to my friends list?
When you sign in with your own twitter or Facebook, it's kinda cool. The numbers on the side will shift and one will be added.
Once there are 100 people, on the set date, Thunderclap sends out all of the tweets, facebook posts and tumblr at once. And the hope is that other friends will share and retweet.