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

All users will be listed in a treeview on the left. All clients will be
listed on the right. To assign a client to a user, first click the
user. Then select the desired client and press the < button
to assign it to the user. When clients have been assigned to a user
they will show as sub items underneath that client. To delete a
user/client assignment click the assigned client under the user and
press the - button.
Note that changes are made in real time. You cannot cancel user client assignments you have made.