Normally there is no need to cancel a Pay By Group -- if the group fails to come together by the deadline, it will simply not charge anyone and stop emailing people after the deadline passes.
Canceling a group mainly exists to send a message and let everyone know the event is permanently off. Canceled groups cannot be recovered.
If you're the organizer of the group and wish to cancel it, you can find the cancel button here:
Clicking this will give you a dialogue box where you can tell your group what's going on. We encourage you to use this in order to put everyone at ease.