How should my deisgn be for a group-messaging app

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Java

Android

iOS

I would like to create a real-time group messaging application according to some data found. For each different section found (lets say for each different sport found) i want to create a group of that sport, and only let the people who are playing that sport join the group.

I am not sure how the design will work for this. I will implement this app for both android and IOS, so they will need to share data, thus i will need a database to store messages.


Can someone lay out an overview of the things ill need to implement this project?

Thank you

What I have tried:

I have not really tried to implement this yet, want to think about the design first.