No, not usually. Mostly, you need to own the core game. There are some episodes of games that you can play without owning the original, but they are separate games, not add-ons.
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You add friends on the game center, then you restate your iPhone and ur friends bros will show up as bros in our game... Add me "vully"
You use it and try it. If the game has special gear you can use it but if the game has no rules you can play with your gear. Find me im the thebeastnoob @edenis4 twitter.
Spore GA is an add-on for spore. It is the same game, but it has a separate desktop icon.
I'm not exactly sure what the question is asking, but, if you mean how do add more accounts onto the game, so two people could play and not just one, you can't. You'd need to buy another game.
You can do this by typing the game ID at the white box beside the "add game" then after doing it, you click the "add game" button(Note: You must own the game before adding the game ids!)