1.In the user-Defined workflow section, under allow external user to participate in workflow, select Yes. External users will be notified by e-mail when those users are part of the workflow and are granted access to the workflow attached web part(s).
2.In the user-Defined workflow section, under allow external user to participate in workflow by sending them a copy of the document, select Yes if you want the document sent to the external users by e-m
3.You need to enable anonymous access for the site the external users must access for the users to access the task list part.
4.By default, workflow can include external users who do not have site access. Create the domain user account for these users.