How can I run a FB ad campaign to promote an FB group?

I wanted to run a facebook ad campaign to promote a facebook group, but I could not find any option to do that. I can only send traffic/reach/brand awareness campaigns to facebook pages (not groups) or to external urls.

Of course, I could do one of these two things:
1.) Run a campaign for my page and then invite the people who liked the page to the group, but that would mean a lot of work as I need to select the invited people one by one, and there is a limit on how many people can I invite as well, and not everyone would accept the invitation.

2.) I could setup a bit.ly url and redirect the external traffic back to facebook, but that would make my campaign more expensive as facebook would see the traffic leaving their site unlike with the likes that stays within their platform, therefore cheaper.

So...
Did I miss something?
What is the best way to promote a facebook group?