It's probably the "invite" button description that I noticed before. There was some reference to that button showing up on the user profile; maybe that reference went away during the recent site software update.
As for how it would work, I think like any community software, a current user at a certain level could enter an invitee's name and email address, and the site would generate an email message to invitee on behalf of the user. The system would create a streamlined account set up for the invitee, perhaps with some provisional permissions for posting to the forums -- sort of a "30 day trial" account that could be easily converted to a full account at any time during that period.
There are lots of enhancements that could be added to this feature, but I think the above is a good minimal set to facilitate and encourage new users.
In the meantime, I'm just going to send an email to the people I was thinking of inviting when I started this topic.