There is one-way synching with the in-game calendar through use of the GuildRaidSnapShot mod.
This is due to limitations of the mod capabilities such that they cannot programatically create new events. This ultimately means that should you decide to use the in-game syncher, events must be made ONLY in-game, and signups must only happen in-game, at which point they can be synchronized with your website calendar. There is no way around this limitation if we wished to use the official in-game calendar. The only possible option is to create a completely new mod specifically for the calendar, but this would require everyone in the guild to have the mod (like the GroupCalendar mod).
But check out Admin > Schedule > WoW Calendar Importer.
Our "Guilds" use the Group Calender Mod, it is a required AddOn. I would like to see a why to UPLOAD from the site to my "GroupCalender.lua" file, so that any changes made on the site can also be reflected in the game.
As a note: We use Group Calender v4.5 because v5.x is buggy.