Any RailsGame will have the standard actions "login" and "logout" defined. Other actions are defined by the game server, or by the game server along with the client JavaScript, the Rails server, or both.
For actions that you'll always allow, it's usually best to include the JavaScript to send or receive them in the JavaScript you serve up statically in your RailsGame view. That way the client can request it (perhaps with a link or button) and receive the JavaScript function calls in response with minimal management on the game server's part.
It's possible to write JavaScript on the client side to send any action, and you don't control the client. Remember, tools like GreaseMonkey will let them send anything. Never trust the client code! So even if you don't include JavaScript to send you an action, always verify actions that are received via Juggernaut. They can be faked!
It's also possible to automatically generate action names and parse them on the client side. I'll leave that as an exercise to the reader -- the code to send and receive them is pretty easy, but you should only do that if you really know what you're doing!