Drop in works by the name of your Echo devices. For example, I have several Echo devices around the house; One in the living room, one in the kitchen, one in the bathroom, etc. From any one of my devices, I can say "drop in on livingroom" to listen/speak to the living room.
Let's say you named your Echo in the FL home "beach house". From your other devices, just say "drop in on beach house". You can set/change the names from within the Alexa app.