When someone you follow shares a live video, their profile picture will appear at the top of Feed with a colorful ring around it and the word Live. Tap their profile picture to view their live video.
While viewing someone's live video, you can tap Comment at the bottom of the screen to leave a comment, and 'heart' to like their video. Anyone who's viewing the video can see when you like or comment. You can also tap the screen to hide comments and likes.
Keep in mind that when you start viewing a live video, the person sharing and anyone else who's viewing may be able to see that you've joined.
Note: Live videos aren't available to everyone yet.