Facebook is testing a new feature to let users share events that they are attending on "Stories" so that friends can also join in and enjoy events together.
You will see new option to "Share to Your Story" when you visit an event's Page on Facebook. When you share it, friends will see a tappable sticker within your Story that also has the event details. Your friends can then also respond that they are 'interested' in attending it.
Your friends also can tap on the sticker in the Story to visit the event page. You can also start a group chat on Messenger with friends who responded.