Users can be added by selecting manage my domain from the user drop down menu, users will then appear on all assigning drop downs.
There is an icon against each item for adding tasks, simply press this to bring up a box for entering the Task Summary.
To Assign an existing task to a customer / project / job, edit the Task and go to the tags box, start entering the name of the item you wish to assign the task to and it should appear in a list below the tag.
Please see Actions documentation.
Please see API documentation.