whatever you are comfortable with. You do not have to publish it regularly.. there will be times when it might seem appropriate.
You might consider posting the list and invitation to sign up the first week of every month.
Think about it. We don’t necessarily PING everyone to these.. but have them posted early in thread to be found. People do get annoyed when they get too many pings
It comes in handy when there is an incident happening in a state. Weather, etc. We can check on those we know.
Once a month or when we have those problems seems like enough.