Syntax: /tell "username" "message"
Example1: /tell "Joe with Spaces" "Message with Spaces"
Example2: /tell Donny SingleWord
I admit it's a bit unintuitive. A better means will be introduced later.
Also, there is a glitch where the sender cannot see their own messages.
Additionally, /tell will refuse to proceed if the user does not exist.
There is no invite system yet, so you or others will need to inform the prospective guest of the chat room name.
Can attempt an obscure or long chat room name to avoid discovery but that can succumb to bad luck or an excellent guesser.
Also can try to keep it on the low down.
Eventually, public chat rooms will be obvious (a list of chat rooms much later on). There will be private rooms added around then.