From my experience, people don't actually do guild quests together. They speak up in guild chat, ask for one, then have it popped if an officer is on. Any and all guild missions can be completed by one player that is prepared to complete them, and its usually the more efficient way to do it. That is outside of areas with mobs you can't instantly kill which I don't believe shows up in many of the guild missions, if at all. The mechanic doesn't require the members that are online to be active, they just have to be online. It is very annoying since the only reason this mechanic seems to be there, is to be there. For me, who's in a small guild, I'm not allowed to come into the game at off hours and just do a mission to help my guild out. Its a guild that needs all the help it can get from its members, so just looking for ways around the mechanic when the conditions can not be met.