Guild Level 20

Do you have to be in the guild when they hit level 20, or do you suddenly gain access to it if you join up with the guild after they hit it.

If you join the guild when it's level 11, you have to raise your own reputation with the guild to Honored before you can purchase and use the Bind on Account armor.

To earn rep with the guild you would have to do activities that increased the guild xp (presumably while wearing the guild tabbard)

You don't need to be wearing a guild tabard to earn guild reputation. Anything that gives guild experience

Pure speculation here, but I believe it will be a two point requirement. First the guild you are currently in must be of an appropriate level. Second your reputation must be high enough to purchase the item. So a toon joins a guild. My guess is that all guild reputation previously earned (with another guild) would be removed and your rep will be set to 0/3000 with the new guild. To earn rep with the guild you would have to do activities that increased the guild xp (presumably while wearing the guild tabbard) Even more speculation is that the various rewards will be available based on guil

On second look the Heirloom gear does not have any restrictions and is account bound, so it looks like all you need in one toon in good standing rep wise with a guild with a high enough level. Distribute from there. -Joel