I'm the guild master of a growing guild in the beta. We got enough people for a guild bank and a guild store during the last beta event. I noticed that, as guild master, I could only toggle access to the guild bank on and off. I could not limit the number of stacks a particular guild member could take out per day. This means that one malicious person could, conceivably, clean out the entire bank.
I put a thread about this over at Tamriel Foundry also, and the people there are pretty agitated about it. I felt strongly enough to send Zenimax an email and tweeted them about the vulberability. I'm hoping this gets changed. If you also feel strongly, please contact Zenimax.
Guild Bank facts: There are 1000 slots.
Guild Store facts: Each player has 30 slots to sell in the store. The listings stay up for 30 days. If a member leaves the guild, their sales are automatically cancelled and their goods returned to them. There is a "house cut" taken from each sale, but this money does not go to the guild. (At least at this time. There is some confusion between the beta server and the PTS as to whether this will be a function in the future.)