If you know anything about LUA coding (don't mean for that to sound insulting, its just a hypothetical suggestion), you can probably look into one of those addons that says Gratz when a guild member gets an achievement. Just edit the trigger (dont know what it looks like - probably just if(gmember.getsAchievement()) or something like that. i don't know that much LUA to write something off the top of my head) and edit what you need it to say. Just look for a string "Gratz!" in there somewhere, the if will be right above it.