How to invite non-community members to a Group

I'm looking for alternative ways of doing this.

One option would be to email a coupon code to non-members and ask them to join the community and then join a closed group via a custom page with a coupon field and a join group button. But Lithium tells us there's no API call to join a group, so the custom page idea won't work.

Any other suggestions?

Use case: We can usually get a lot of attendees to our annual customer conference to join our community by offering them attendee-only content via a private group. In Jive we can email them an invitation to join the private group, and we're trying to replicate this in Lithium as part of our migration.