My new user can now send a message to discobot when I did both of the following
enable personal messagesenabledmin trust to send message1
However, my question now is that I need personal messages disabled AND new users to be able to go through the discobot tutorial. 
Discobot is basically in the same line of system, I think. Basically everyone should be able to do its tutorial, right? Regardless of if they can generally send messages?