Edit plugin to restrict to specific membership group

I have a free trial membership group, and then various paid membership groups.
When someone joins the FREE group only, I wish to send the automessages. These messages are about the upsell to paid memberships, so I only want these to go to the trial group. What do I need to edit, in order that the automessages only post to that specific membership group? Thanks so much for your help and awesome plugins,