Instagram just keeps adding features that seem trivial to us, but might come in handy for the more aggressive social media users out there. Case in point: this latest feature allows for you to share a friend's story as your own story if you've been @ mentioned in that friend's story.
A notification that you've been mentioned would already be sent to you via your DMs with the friend who's mentioned you, but there's now an option to "Add This to Your Story." Press that, and you'll be able to modify the photo with stickers, reposition it, etc. To your followers, the story will appear with a smaller image and a colored border, and they'll be able to tap your friend's profile as well.
The update is part of Instagram v48 on both Android and iOS, so it's out now. Note that only public accounts can take advantage of this feature.