Tasks rules give an option to assign tasks to your teammates automatically or manually. You can also create a task and assign a teammate later. It is also possible to manually change the teammate assigned to the task.
To assign a task manually:
Go to Calendar.
In the top right, change to Task view
Find the task you want to edit and click on it.
Select a teammate from the list
You can also access pending tasks for the Operations dashboard (if they have never been assigned to a teammate before):
Go to Operations.
The Operations dashboard opens on a list of unassigned, pending tasks. You can click on Edit and assign a teammate from there.
Assigning a teammate to a task will trigger any "New task" notifications that are relevant for the task.
Changing the teammate (un-assigning a teammate and assigning a new teammate) will be triggering "task deleted" notifications to the former teammate, and triggering "new task" notification to the new teammate.