Task’s responsible person, participant or observer can be selected not only from existing Bitrix24 account users, but also added as an external user via email address. In this case the task will be shared with this user by email:
External users will get task description as email message and will be able to participate in the task by replying to the email or by clicking the link inside the email message (task details will be opened in a new browser window).
All comments and files added to the task by email or via the web page will be automatically attached to the task.
Read more: How to create tasks from email in Bitrix24