The textbook definition of "externality" is where some activity has an effect (whether positive or negative) on people who are neither party to that activity, nor in a contractual relationship with those people.

So, creating a meetup group that other people will enjoy has a positive externality, but note if SilasBarta had been hired by those people to create that group there would be no externality (unless it also benefited some people who hadn't hired him).

As for the reference to counterfeiting, that I believe is (based on previous discussions with SilasBarta) a sly reference to Keynesian economics, and you should probably leave it to one side if you're still trying to get your head around externalities.