To restrict your employees to access all the activity items, you can set the restrictions in the Manage Access screen. This allows you to assign certain activity items, expense items and projects to each employee and it applies throughout CORE.
Open the Manage Access screen from Settings > Access & Permissions. Select the employees and assign the relevant activities to each of them. The employees will be able to view only the assigned activities in the drop-down lists and on screen. Check CORE Help for details.