Tasks and workflows can be assigned to any group member with the role of contributor or manager. They can only be assigned to a single member at a time.
If a task or workflow doesn’t have an assignee, it is unassigned.
You can change the assignee of a task using the comment dialog. Similarly, you can assign a document workflow in the workflow dialog.
When you re-assign a task, if the needed member name doesn’t display in the drop-down, you can start typing the name in the Assignee field and select a name from the drop-down.