It would be difficult to encrypt a message on a list that could be
read
by everyone because GPG is generally asynchronous encryption. You could
all share a public/private key set which would work but that's an iffy
solution as well.
So it seems there's no way around it apart from individually sending
to list users. But this may become more cumbersome when the lists
expands and the subscribers become too many. I guess that would do for
now.
Thanks