I don't have a ping list, but from what I remember posters saying is that some of them keep them in Word or WordPad, it's just a text file that lists the FReeper names, each separated by a semi-colon (;). Then, when you want to 'ping' the group to your new thread, after you post it, hit the Reply button and copy/paste the ping list into the To: field at the top.
That's the way you keep a ping list. I kept mine in notepad, and would copy & paste into the "TO" field when I was ready to ping.
It would also be best to have people let you know that they want on (or off) your ping list instead of just adding whoever you wanted.
Hope this helps.