To add task assignees open the task by clicking on the task subject while the tasks are listed.
On the right side of the task modal there will be dropdown to select assignees.
You can add unlimited number assignees to specific task and track their time logged.
After you add member as assigned to task this member will get notification email and built-in that is assigned to this specific task.
When task is related to project only project members will be shown in the dropdown while selecting members as assignees.