Thanks @codinghorror - that is the invite button I was referring to above. As discussed elsewhere, this functionality is not yet working for private groups but is being worked on. So I was going to wait until the next update to try this again and learn more precisely how it works. But yes, I think that invite button might well be the “gateway drug” to entice mailing list folks to come on over and join discourse forums. 
The invite button only really works for inviting one or two people, however, and does not really help the listowner who wants to move a mailing list over to a group on discourse because they won’t necessarily come. This is happening to me right now on Kabissa… I have a core team of about 15 people I am trying to entice over to help me learn about discourse, and so far only a handful have moved over on their own!
Ideally I would want to impose this on these 15 people and just import them to discourse and add them to the group. This is a common requirement, methinks, and the functionality that would make this easiest for most people is a CSV import, much like Mailchimp, CiviCRM etc and other tools that folks are familiar with for managing contacts.