Steam supports using multiple game library folders by default, you could just create a folder which all users can read/write to then add this folder as a library for both users. Both users will have to maintain a separate copy of the Steam client and update it, the library includes the games themselves only.

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You could create a main folder in a read/write location and have a symlink at ~/.local/share/Steam to this folder, which should work hypothetically, but I haven't tried that. This will all users use the exact same Steam files.

You can also have a look at bindfs, note that it is not nearly the "most convenient way"