The intent of the IOnlinePartySystem interface is to expose the party/lobby functionality of each available online platform.
Currently the only party system implementation is via Epic online services and we have not released that code at this time. The code is not very useful unless you have the hosted services to connect to. Having said that, the party implementation is primarily dependent on using an XMPP for signaling the invite/join request/response processing, the data broadcast between party members, and the chat functionality. If you can stand-up your own XMPP server then we can provide the code for you to use.