There are 3 distinct areas you can configure, based on your needs:
By default the timesheet shows all clients and projects to the users
You can limit access to clients and projects by assigning them Exclusive members
If you do so only members will see them in their timesheet.
You can define some users as managers.
By assigning them a team of people.
Or the projects they manage.
This give managers access to the time people entered in their reports and for the approval.
To add a team to a manager. Go to the page of the manager.
Locate or add the module Team.
To add projects to a manager. Go to the page of the manager.
Locate or add the module Project Manager
When you click Customize.
Several screen modules are available(Timesheet, Report, Approval,etc.)
Each user is assigned to a user group by their User Details
These are Employee, Contractor, Team leader, Project Manager or Administrator.
Under Settings >> Authorizations, you can define what modules a user group can see.
Authorizations, only give the rights to some modules.
This does not tell the system which data they can see.
Note: All these areas do not apply to Administrators.
Administrators have access to everything.