Users tap a new icon on the bottom right of the screen and tap "Add" to invite anyone that is current watching a broadcast. Once that user joins, the screen is split into two and other viewers can still like and comment. The invited guest can be removed and others added at any time. These live videos can even be shared to stories when the broadcast has ended.
The feature is being tested with a small percentage of the Instagram community and is expected to roll out globally over the next few months.