Correct, with the free account you can only add a track to one group. The pro plan(s) allow adding tracks to up to 75 groups.
Edit: The main benefit of upgrading is that you can re-upload/edit a track without losing all its stats and comments.http://help.soundcloud.com/customer/portal/articles/247820-what-s-the-difference-between-each-subscription-level
Boardwalk Empire, Breaking Bad, Dexter, Nurse Jackie, Frasier, Cheers, Outnumbered, Corner Gas, Big Bang Theory, Hi-de-hi, You Rang my Lord,..
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Happy birthday Envato!