Minor update on this. It turns out that requiring new members to make
a request to join the google group also requires the new members to
get a gmail account. That's just annoying enough to make it a non-
starter, so here's plan B. Everyone and anyone can still join
(including spammers), but all messages from newcomers are held for
moderation. That gives me the chance to evict any scoflaws and
prevent their spam from reaching the masses. Existing members can
still post as they did previously, and new members can receive
messages immediately; they just can't see their own posts show up
immediately. I think it's a reasonable compromise.
Also, everyone who was a member before is still a member, so there's
no need to sign up again.