SharePoint groups that people can belong to.
SharePoint permissions are important too when it comes to using Excel Services. Why? Because most likely, you want people to look at data in Excel Services, and you don't want them editing the actual worksheets in the Excel program. The best way to prevent people from editing is to make sure permissions are set up properly for the site.
Unless your organization has permissions set up differently, your SharePoint site has four groups which people belong to: Owners, Members, Visitors, and Viewers.
People in the Owners group can do a lot. They can set up site-wide settings, library settings, and so on. They can also add and edit files in a library.
People in the Members group have less power. They can't determine settings, but they can add and edit files in a library.
People in the Visitors group can do even less. They can only open a file from a library and see a read-only copy. But they cannot edit that file and save changes.
People in the Viewers group can do the same as Visitors: they can open files as read-only. But there's also something about the Viewers group that makes them special. We'll cover that in a moment.