In
Project Clock, tasks can be assigned to users. When tasks have been
assigned in this way, users can only enter hours on tasks to which they
have been assigned.
To assign tasks to users select Tools | Manage User Tasks from the main menu. The following screen will appear:

All users will be listed in a treeview on the left. All tasks will be
listed on the right. To assign tasks to a user, simply select the
desired tasks and drag them over to the user. Multiple tasks can be
selected by using the Ctrl and Shift keys. When tasks have been dropped
onto a user they will show as sub items underneath that user.
Note that changes are made in real time. You cannot cancel user task assignments you have made.