Alternatively, is there another technique to allow a “set” of users send PM to another group or “set” of users?
My initial question was about groups, but I was wondering about some way to utilise trust_levels, or custom fields of users (that is my notion of “set”).
My problem is that I need to allow one some users to PM some of the users (experts). I was thinking to have a post guiding the users to visit an external web site, do some integration with 3rd systems and using the Discourse RESTful API to “update” the user in order to have now the right to PM the expert user set.
Also - since my Ruby skills are zero 
(so no hope to develop a classic plugin), another option for me to integrate would be custom JavaScript on the web site theme…