Currently, the only possible ways of cross-communication are through data stores or using web requests. Although, both of these methods have limitations to how much you can use them. HttpService has a higher amount of requests you are limited to, however this would require you to have your own domain or website that it interacts with. However, with both of these methods comes delays while posting or getting information (messages). Assuming you decide to use HttpService, you could tell the player when they have exceeded the amount of requests in a minute (maybe 5 - 10 messages in a 10 player server). There is also a release note that has promised a MessageService which is dedicated to cross-server communication.