What exactly does it say when you ' re trying to accept a meeting ? It sounds like they might have readonly access to this calendar . The second issue doesn ' t sound like it has anything to do with permissions . It sounds more like a client issue or a connection to the server . Can you pinpoint it to happening on 2010 or 2013 ? Squimjay is definitely on the right path with cached exchange mode , but test it out on an couple users first to see if it fixes the issue before modifying the entire environment with a GPO . You can get to that by going to account settingsopen mailboxchecking used cached exchange mode . I imagine you won ' t be able to try anything until next week , but if you report back with more info , include if it was on or off to begin with .