If you need to have certain events available to one location or site and another group of events available to a different set of volunteers, you can use groups. First create a group under Users and then Groups. Add a new group (like Location A). Then add your users to that group. When you create an event, you can easily add that event for that specific group if you go into the group, and find the nav bar Events. Click to add an event and enter the event information. Adding an event from the group will automatically link the group so only users within the group will be allowed to see that event.
You can also update the privacy on an existing event to only be allowed to be viewed certain groups. First go into the group. Find the privacy (Public, Restricted) and click on it. There you can add the group you wish to allow to that event. Only privacy of restricted will limit access to users or groups. Public will allow anyone to view the item.