Instagram is testing a new feature that will let your friends add to your posts. When you’re about to post a carousel, Instagram will give you the option to let followers submit their own photos and videos that they want to include.
The submissions won’t get added to your post by default, as you’ll have to approve each photo and video before they’re added.
This is a pretty neat idea. Not sure I've seen this before on social media apps. But I could be wrong.