I think pinging other FReepers just means typing in their names in the "To:" box when you're composing a reply. You separate their names with semicolons.
Some FReepers have long list of others who are interested in a topic, already prepared and ready to cut and paste into the "To:" box. (Saves a lot of typing.)
That way when people check their "my comments" page, they'll see the article 'pinged' to them.