There actually IS the capability for members of your site to set their guild affiliation. Check out "Characters and Roster" and check the box next to "Players can specify which guild they are a member of"
And there you have it
Yes,
But how can I add that field as part of a snapshot / upload?
oooooh...
Hmm...I see. So you'd like the GRSS snapshots to track guild then, like they do Zone, and when a character is created on the site, they get initialized to that particular guild?
I'm not sure how feasable it is at this point, but given the WoW mod interface, I suspect it'd be a pretty painful process.
....Yeah, I just looked, and tracking the Guild of the members would be a fairly noticable chore, since the raid info doesn't retrieve the guild info, it would require performing a /who for each member of your raid, and because of the built-in /who delay, even with trying to optimize the process and reducing it to only like 15 queries instead of 40, it'd still take over a minute to take a snapshot. On a busy Raid night, where other guilds are raiding the zone you are in, it'd easily take more like 30 queries, so you're looking at 2.5 minutes for snapshots.
Your best bet is just to have the people on your website specify their current guild, as that only needs to be specified once.