Party Play for Streamer.bot allows you to communicate with other Streamer.bot instances that are not on the same network and belong to other people.
Party Play for Streamer.bot uses Twitch OAuth to authenticate you as someone with access to this service. You will be then allowed to connect to a custom messaging system that allows multiple Streamer.bot instances to communicate with each other without being connected to the same network or using the same Twitch account.
Party Play for Streamer.bot is considered middleware - you will still need to sort out the logic of sending and receiving messages yourself or have someone help you do it.
While completing the Twitch authorization flow is possible for anyone, this application is only intended for use by a small number of creators. If you're not on the list, you're not going to gain access to the Party Play messaging system, despite authenticating with Twitch. If you came here from a web search, you are out of luck, as I have no plans at this time to scale this to be a service usable by the masses.