Homeassistant supports grouping of media players with media_player.join and media_player.unjoin services.
Unfortunately the build in media control card does not support those features and so you have to create a custom UI for that using buttons.
I am using the custom mini media player which does have support for join / unjoin. Unfortunately that project is not very active anymore and at the moment does not support the new Linkplay integration I am using.