Blue poster Hortus says this is not a bug:
Guilds are functionally limited to 500 members.
There is no cap on the number of members in a guild so you can invite more than 500 but the UI will not support it.
This may change in the future but it is not considered a bug.
I briefly had a character on Dark Iron in the Knights of Arcadia, the first of the Penny Arcade guilds <insert fangirl noise here>. The UI issues over 500 members got so unwieldy that they split it into multiple guilds.
Do you think WoW should support the functionality of guilds over 500 members? Or do you think Blizzard's resources are better spent elsewhere?