I'm not sure. It may be that that sort of thing might be possible, working on the same priniciple as "on event" triggers:
If x happens, then y
An example from
this site is the
WarriorRevenge mod, wherein if you dodge, parry or block, it plays a sound, so you know to activate the revenge ability. (Undoubtedly there are other examples here too, but that one popped to mind. Sorry to any other mod author whose mod I didn't use as an example.)
I'm just not sure if "guild member shows up within z area" counts as an "event".
Your best bet when it comes to that sort of thing would be to head over to
www.WoWWiki.com. There is a boat-load of info there, what you are asking about may already be covered there, one place or another.
On that note, I'm actually going to move this thread down to the Developer Discussion area of the board. We've got some users on this site that know more about this sort of stuff than I do, hopefully one of them will see this and can help you out.
If all else fails, try dropping a note on the Blizz UI forum, Iriel (amongst others) are fabulous about helping with things like these.