When you are posting a reply to someone else's post, notice on your screen the "Your Reply:" box, that you are typing into, and above it a smaller "To:" box with the name of the FReeper to whom you are responding.
To ping other FReepers to your reply, just add their FReeper names in the "To:" box, separated by semi-colons. Then those other freepers will see your post when they look at their Pings (also known as "My Comments" list.
It is the popular way on FR to bring some other FReepers attention to an interesting thread or post.
I think I can handle that. Thanks for the instructions.