It was a bit fiddly. You have to make sure the app is updated to the newest version. When its updated it'll ask you your name, and contact number (use your mobile number). It will take then the contacts from your address book to see who has an Alexa account. For me, there were none, none of my friends with Alexa have it enabled. In the app, click on the middle button at the bottom (conversations). From there you can find drop in, it then listed my 2 alexas and I can choose which one to drop into.
Does that make sense ?