For whatever reason, Lucy's iPhone wasn't connecting to the guest network at the office. I tried a couple different passwords, which was failing, and then I wanted to verify it on my phone. And lo and behold, I get this prompt: Do I want to share the password for the Guest network to Lucy's iPhone? I answer yes, and Lucy's iPhone gets on. That's amazing. Unexpected. Almost magic. I assume it's because we're in a family account, but still, that's cross account WiFi password sharing.