+11 on this.
The problem with triggering a Bot session by sending a PM is that the user needs to know how to send a PM to discobot so that discobot can trigger the tutorial about (amongst other things) how to send a PM.
Many of our users took a while to figure out PMs, it was one of the first User Guides I wrote.
What new users need is a place where it says:
Blah blah, blah…
To start the new user tutorial click here
A list of all user tutorials is available here