On a computer keyboard, press Shift + Tab.On your Stadia Controller, press the Stadia button.
You can find and join public parties on your social panel: Note: If you're using a supervised account, you can't set up public parties or join them without being invited. In addition to parties, you can invite other players to multiplayer games that have online multiplayer modes. Parties are available on your TV and on, and will be coming soon to supported mobile devices.
You can connect a microphone to your device, Stadia Controller, or computer to communicate with each other, and send a game link to party members so you can play games together. Parties let you communicate with other players via voice and text chat even when you aren't currently in a game.