At checkout, when choosing the details of the game (duration, number of players, date), you can also select if you want the game to be public or private.
If you have selected it to be public, anyone in the Kickr App community can join. If you select it as a private game, you will need to send your friends the link to join the game by sharing it.
The sharing link can be found both in the Reservations -> Created Games area and on the Thank you screen when creating the game.