I've been watching this thread for a couple of days, and am surprised it hasn't gotten more attention. Klepsacovic of Zul'jin pointed out that guild ranks are listed on the armory, and the OP should have checked there. Some people are in the habit of checking the armory for any potential groupmates? I am not, then again, I don't check guild tag either when PUGging. To me bad players can come in many flavors, tanks that don't pay attention to mana, low DPS that are not interested in constructive feedback, loot ninjas, unpleasant people, and anyone who refuses to follow directions or kill order.
I generally do not decide who to invite based on guild tag, but it has been a factor in not inviting players before. My guild has players of all skill levels from the young lady I used to work with who's still learning the game to raiding relics from beta. We try to help folks not just in getting gear, but also in learning to be self sufficient.
I expect my guildes to be competent and courteous when in groups. Skill and gear come with time and practice. I'm not willing to stop playing with my friends because they're on a different level. I do struggle with ranking guildies who are friends but not quite up to speed. We do not have a dedicated friends and family rank.
Is Necran of Mannoroth correct? If you have bad people in you guild are you a bad guild?