For many-to-many communication, we use publish/subscribe, which is a messaging pattern. This is where senders of messages, called publishers, do not program the messages to be sent directly to specific receivers; rather, the receiver needs to subscribe to the messages. It's a highly scalable pattern as the two are decoupled:

Asynchronous messaging allows a service to consume and act upon the events, and is a very scalable pattern as you have decoupled two services: the publisher and the subscriber.