That's pretty straight forward to do.
From the client, execute a function onRegion (without any filter keys so that it executes on every member hosting that region).
In your function do something like this:
public void execute(FunctionContext context) {
RegionFunctionContext rfc = (RegionFunctionContext) context;
Region r = rfc.getLocalDataSet();
// count entries or whatever...
}
Unfortunately, from the client there is no easy way to retrieve all the connected members.