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Multiplayer URLs have “join-“ in the path, and are available from the “share” button in the top left.

It would be very handy (and intuitive) if all REPLs were a live read-only window on what the owner was typing. The reload-page-to-get-latest-version thing works, but multiplayer is there and awesome so it seems odd to hide it with a different type of URL that looks almost identical to the regular read-only fork-on-edit one.