That's a good idea! Discourse automatically sends a welcome message as a private messages when a new user signs up. Maybe that is a good place to start?
EDIT - below is the content of that message:
Thanks for joining P2PU Course discussion, and welcome!
This private message has a few quick tips to get you started:
There are no next page buttons or page numbers – to read more, just keep scrolling down!
As new posts come in, they will appear automatically.
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Enjoy your stay!