Right - so just glancing at the code, the number of guild members being too high is exactly what's going on. It can't fit the roster into a single message, and hence (as a safety measure) simply bails and doesn't do anything (even though it would likely still succeed, since not every character has the maximum number of letters in their character name).
Now, I don't want to artificially limit your guild growth, but 737 members really is extremely high - just scrolling through the roster must be a pain. I could change the protocol to allow more members, but I don't think that's the right way to do it. There has to be some limit, and it seems to make sense to make it a bit lower than what's required to fit into a message.
So, what I'll do is fix it so much that the guild will load for you - as I said, for safety, it's being pessimistic, and bails the moment it sees it can potentially not fit the whole roster into memory. I'll change it so it tries to fit as many as possible, and only bails if it actually runs out of space.
Nonetheless, I will also put a limit on the maximum number of players in a guild - with 650, there's no risk into hitting that limit, so that'll be the limit for the time being. This will be a soft limit for a few weeks, so you have some time to think of how / best to get to that number. After that, it'll become a hard limit.
This'll be effective next reboot, let me know if there's any concerns with that.