Letsgodoit is a button that users click on websites of events they want to go to (e.g. sports events, concerts, movies etc).
Its kind of like a modern "Email to Friend" button that sits where the current email to friend sharing button goes.
Once the user clicks the button they select the friends they want to go to the event with. Letsgodoit invites the group and makes it simpler to organise to do the activity by streamling the conversation and reminding the group as the event approaches.