The setting is not in the Notification Center, it's in the calendar app.
Go to Calendar > Preferences and highlight the Alerts tab. Now put a tick in the checkbox labelled ""Turn off shared calendar messages in Notification Center"
To be honest, I saw this setting before, and the fact that it's a double negative (you're putting a check mark in a box to disable a setting) consfused me, and I left it unchecked.
By the way, there's a similar setting on iPhones/iPads too, and again it's hidden away. Go to Settings > Mail, Contacts, Calendars and scroll right to the bottom. Where it says "Shared Calendar Alerts" change this setting to "OFF" (it's ON by default).