Users can choose to Share their Calendar Events with other Users, or a System Administrator User can login for the User, and set their Calendar Event Sharing settings for each User individually.
NOTE: Users can do this themselves, or an Admin can change these settings.
Logged in as the System Admin, navigate to Setup
Setup > Manage Users > Users
Click on the Users menu and choose the Active Users List View
NOTE: The System Administrator can Login as any User.
Click Login next to the User you wish to assume their identity and share their Calendar.
Once logged in as the user, navigate to Setup and search 'Calendar Sharing' and select. Click 'Add' and here you can select who to share the user's calendar with, options include: sharing with a Public Group, Role (if Roles are setup in your system) or specific Users.
Use the Add button to move Groups, Roles or Users into to "Share With" column.
NOTE: Make sure to leave Calendar Access on Full Access. This assures others can Edit and Delete Calendar Events for this Users.
Save your changes
This enables full access to the user's Calendar Events.