Another option is to add an email address as a staged user, and then go to edit the staged user email preferences to turn on mailing list mode, or to set certain categories to watching. You can also add the staged user to a group giving access to private categories.
I see this as a hack.. so I would use it with caution and only when you are dealing with people you know well and who you are certain actually want to participate in this way even though they don’t bother to sign up.
If a case study helps.. we have done this with working groups that exist already outside our forum. The people convening the working groups want them to use the forum and everyone has agreed to it. But there is always the straggler or two who for whatever reason ignore all the invitations to join and participate via the forum. With this approach we don’t have to have a hybrid system where we use the forum to work with most people but then follow up separately by email with that straggler.