It might not be guild progression, but I had some personal progression. Pepper asked me to join a PUG for Siege of Orgrimmar, so I decided to give it a go. It seemed like it was at least half of a guild run. We started at Galakras and made it through the Dark Shaman. It was my first time downing them!
It was actually a rather unpleasant experience. They were very rude to non-guildies. I stayed simply because I thought I had a chance at progression. Is that bad of me? Whatever.