I am also working on this right now… I agree with @codinghorror that discourse is generally ready to start a new forum instead of just using a mailing list tool.
However, one feature missing in discourse right now for drop-in mailing list replacement is indeed the ability to add subscribers or allow them to subscribe just using their email address. They also need the full name and username and password:
I guess you could complete that signup form on their behalf and make up a password if you know their name and email address but then they would still have to click the link in the email to verify their email address to start participating, and somehow get their password.
There are also some cultural differences between forums and mailing lists and how they work that I’ve been grappling with that you might also have trouble with if you are hoping to seamlessly replace say mailman with discourse.