Instagram's had live video since December 2016. But back in August, Instagram revealed that it would be adding the ability for users to invite a friend to their livestreams "in the coming months." Well, it's been a few months, and the company has delivered on its promise. 'Go Live with a Friend' is out now with version 20 of the Instagram app.
The feature is pretty self-explanatory; when you start a livestream, you'll have the option of inviting someone via a little icon with two faces. Select their username, hit 'Add,' and the invitee will be able to accept or decline. Your guest will be able to leave at any time, and you'll then be able to add someone else. These special streams will be identifiable from the two stacked circles in the 'Stories' section.
Version 20 of the Instagram app on both the Google Play Store and iOS's App Store brings this functionality. I just updated through the Play Store and 'Live with a Friend' is indeed live, but if you can't see it for whatever reason, you can grab v20 (or even v21) of the Instagram app from APK Mirror.