Unfortunately, Wattpad is currently unable to support co-writing. We do not have any features allowing more than one person to collaborate on the same story.
While some Wattpadders might share their accounts in order to collaborate, please keep in mind that we can't support shared accounts in the same way we support normal accounts. This is because the two pieces of security that allow for access to any account are the password and the email address. If you share both, it makes it very difficult for us to help you while maintaining the security of the account. You can read more about this in the Shared accounts article.