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 document workflow does not 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 does not display in the drop-down, you can type the name in the Assignee field.