The trading functionality in Pokemon Colosseum is unlocked right from the start. You can trade Pokemon with other players who also have the game, even if you haven't progressed very far in the story.
However, you will be limited in what you can trade until you reach certain points in the story:
* Early Game: You can only trade Pokemon that you have caught yourself.
* Later Game: Once you progress further, you gain access to trade features like trading with NPCs, which might offer special Pokemon.
So, while beating the game isn't a requirement to trade, it does open up more possibilities and benefits for your trading experience.