As Harvin noted, you can't have external access.
However, you can list events in the guild message of the day field (which would also work without an addon).
You could create an addon to help a guildleader or officer with keeping that up to date:
- Define events beforehand (including recurring)
- The addon updates next 3 (or however many. I don't know max length for field) events in field
- Addon auto-removes old events.
If you had the events listed in a certain format (either above or created externally), you could create an addon for users that:
- Displays upcoming events at login
- Displays say a 15 minute warning for events
Not nearly as efficient to update as if you could pull from a guild website, but still stuff you could work on.
Edit: Also, seen the guild mailing addon used for sending out notifications of upcoming events, for something that exists currently.