Actually, it will not be possible to go back, but you could register more than five signatories from the outset. We have considered the situation and decided that a petitioner could register up to 10 of their friends. As soon as we receive five confirmations, the petition will go ahead.
There are all sorts of potential situations. Someone may misspell an email address or forget to respond. So we had to impose a limit, and we decided that 10 supporters could be registered.