How to use Amazon's Alexa devices as a home-wide intercom

If your home is kitted out with Amazon Echo devices, you can now use the "drop in" feature to talk to all of them at the same time. This essentially brings a quick and easy intercom to your home. Up until now, you could use Amazon's "drop in" feature to send a voice message to only one other Alexa-powered device. Let's say from the Amazon Echo in your living room to the Amazon Echo Dot in your kitchen. You could "drop in" then start a conversation that works both ways.