Turns out there’s a separate global chat function that allows group messages to be sent and received persistently. I.e. I could send a message to Stu right now and if he was a member of the chat he would receive it when he logged in.
I think it’s also at the account level, so it doesn’t matter what character you’re using.
It’s the little speech bubble on top left.
Stick yer character names up here and I’ll send invites to that. It’ll be handy for things like guild passwords etc.