Google is gradually bringing more and more useful features to Stadia, many of which probably should have been available from the outset. It's still ridiculous how long it took to add a search function to the store, for one thing. Shortly after the arrival of free trials for some more titles, Google is rolling out an option to join a friend's game without an invite.
A support page spotted by 9to5Google notes that the option is available in select games, including Far Cry 6. You can switch it on by going to the privacy tab of the Stadia settings. In the Current Game section, there's an option called Join your current game, which you can enable to let others join a multiplayer game without an invitation. You can set this to friends, your friends and their friends, all players or no one.
This is a welcome update that brings Stadia a little more in line with other gaming platforms. It could encourage groups of friends to pick up the same games since it'll be a little easier for them to play together.