It is one of those little aspects I don't like in WoW: Elitism: it is terrible actually.
The very best times I had in WoW was when playing with real friends and trying to do heroic dungeons while whiping constantly as noobs.
In 2006 you had to fill in 10 page documents to be allowed in certain guilds, I bet you a 1000 to 1 we had far more fun with our noob group than these guys dressed in epics and Legendaries standing bored against the walls of Stormwind ...
I know it is not helping much, but as a Guild leader take EVERYONE with you (just set a limit to their gear score) in Flex raiding.
WoW is a game you know you need to have fun with, not some academic title.
After a few weeks, these guys will be so pleased they can join your Flex raiding group and WILL become better. If not they wll realise this part of the game is not for them and they will still support you in mats etc...