1 - Make the message board in the guild one which can’t be picked up by players. At the moment someone could go in, pick it up, share everything to anyone interested. That’s happened before and is the key reason why a lot of longer term assassin characters won’t use the base as a means to meet up with others. There’s no realistic follow up to when something like this happens. It’s such a small thing to just put a message board in there on the module.
2 - The doors shouldn’t allow non-members access. It’s against the rules IC, other secret organisations don’t work this way. Make a conversation object jump you inside rather than a door opened by a key with a transition anyone could go through.