3
the notifications options do this, as stated above....but each user will need to set their own preferences in the 'Edit Account' area of the user menu, so that it will e-mail them and not PM them.
The other option of course is to post a link to a good RSS reader or e-mail app that will pull RSS feeds for you. Then each user can just use the RSS feed to get the forum contents on a regular basis.
(keep in mind that all of these scenarios keep people from coming to your site...so if thats not a problem, go for it!)
Happy xoopsing!