I would like to configure my board so that when someone makes a post or replies to a post they are automatically signed up for instant email notification about replies. Is this possible? If so, how? I can't figure it out.
It's illegal to force people to receive emails. However, you can tell people to go to usercp.php?action=options to turn on subscriptions.
This isn't "illegal" if it can be turned off.
vBulletin can have this turned on at registration (either voluntarily, or as part of the sign-up process and hidden by the admin). Part of the sign-up agreement is to instantly receive emails on new replies to threads you're involved in. It can be turned off if desired by the user, via the profile/UserCP. Most users like this feature.
It's not spam.
My users also want this, on the myBB forum I'm running.
EDIT: Actually, it's already documented here:
http://community.mybb.com/thread-116603....=email+PMs