This is one of those things I like to think falls under forum etiquette (as opposed to explicit rules, which one could very well make a rule specific to adding recipients if they felt it was needed) - I think it’s good etiquette to ask the current group message participants if it is in fact ok to invite others.
I like this idea. Not sure how it would be implemented, like maybe a checkbox like the official warning checkbox staff have? (an allow group pm setting I suppose 
)
hah, Yea I can think of some awkward situations where I was invited into conversations where I felt like perhaps it wasn’t appropriate for me to be there.