You can configure for each user if he is allowed to add tasks. This setting is valid for the web app as well as for the app on the smartphone. You can find more info here (1.7).
If the user is allowed to add tasks his timr web app will look like the following:
No matter which tasks are assigned to the user, he will see all tasks of your timr account in the menu "Administration". The user can add additional tasks on every position in your task structure. You can find here all details about adding tasks.
In the menu "Recording" the user sees only the tasks that are assigned to him.