Play Together is a feature which enables you to start a game directly from the Party screen. You can then manually invite guests to play from within the game.
Games that support the Play Together feature, can be started for all Party members at the same time and in-game party is created automatically.
Go to [Party] > [Play Together] > [Start Game] to create a new session.
Note: You can still select games that do not provide dedicated Play Together support, but you will need to manually invite your Party members when the game has launched.
You can join a Play Together session (Current Game Session) in the following ways:
The host of a Play Together session can send you an invitation to join. Open the Invitation and select [Join].
Go to [Party] > [Play Together] and select one of the Current Game Sessions to join.