You activate access control within the settings for this specific event. To do this, click on the relevant event and move your mouse over Settings, then select Event settings from the drop-down menu.
In the event settings you will then find the item Access Control at the bottom, which is deactivated by default.
Now click on Yes and select the users who are allowed to access this event.
You can also adjust the role of the user if this needs to be adjusted for the event and the associated user.
You can find out more about the different roles and the permissions associated with them here -> USER ROLES & PERMISSIONS.