Unlike assignment, which is about delegation, sharing is about keeping people informed and involved.
In addition to asking someone to complete the task, you might want to share a task with one or more other people. The people you share with can view the Chat, or in the case of the Share task type, can view and edit the data you are sharing. Sharing is available on most task types, even ones that do not involve sending work to a partner.
Set up sharing using the Share option from the task's context menu, which is also available in the People section of Task details.
Use the Share with drop down to add someone to the share. If you are an administrator, you can share with someone you have not connected to before by name and email, just like Assignment. Use Share to share the task.
Use Remove to remove a share. If you are sharing chat, the partner's chat task will be retained, but will be shown as Recalled and set read only. If you are using the Share task type, the partner's task is deleted.
The task is only shared when it has started. Sharing does not show messages before the task started, and new shares only get to see new messages (they do not see previously-sent messages).