I enabled chat, and added everyone at the same time. 
(sorry, I missed it out of the step-by-step. I’ll add it in now)
I think I’ve clocked it @Roman . 
If I add everyone to chat_allowed_groups, everyone gets to access chat and receive notifications, but doesn’t get the @all emails. However, if I add trust_level_0 then the non-staff test-users get emails about the @all mentions.