How can I chase up mailing list subscribers who don't confirm their opt-in?
Taking answer from comment from @eli lisseck... Try using {action.subscribeUrl.gid} where gid is your group e.g. {action.subscribeUrl.5}. More generally, however, beware of assuming that these people have indeed chosen to signup rather than had their emails added by a bot/spammer. Trying to work around the double opt-in will make your site non-compliant with anti-spam legislation in the US and elsewhere, and will make your site more likely to get blacklisted as a spammer.