You become the host and get a code. Send it to your friends (up to 3 players in total).
Enter the code from the host.
Co-op campaign for 1 to 3 players: friends replace the AI allies (three players = no AI allies).
Team Match vs Bots: a team of players against bots on one map.
Online play works only for signed-in people you have shared the game with (the Share button in claude.ai). A public link without signing in is not enough.