Coming from EverybodyPlays...

Although the starter kit gets you access to three separate Play Sets, and although co-op's a large part of the Disney Infinity fun, it's been confirmed to Everybody Plays that if you only own the Starter Pack, you'll only be able to play through the Play Sets on your own. Should you want to play along with a friend in co-op, they'll need to have bought an additional character from that film.

So...you'll need to shell out even more cash to get a friend in on the fun. The starter pack is only for a single-player experience. Disney Infinity is quickly turning into quite the expensive endeavor.