You can have up to 3 party members at a time so you will have to pick the best characters for your party from the list of available characters. (gearnuke.com)
You'll play as one of the 10 available characters in the console version, or one of six in the handheld game. (shacknews.com)
Each can be used at any time when available, and their effects affect every currently available character. (playstationlifestyle.net)