You can ONLY control Stahn.
It is possible to play with your friends if you find "channel rings" ( or something like that ) and whoever equips them can then be controller by the second, third and fourth player. So, if you get all three channel rings you can be four players controlling all four members of the party. But controller one can only control Stahn.
If you need the others to do specific actions, you need to go in the battle menu ( press triangle during battles ) and then you can assign them to cast a specific spell, target a specific monster or use a certain item. You can also change their tactics during battle ( only useful when fighting bosses so that you can make them go all-out only for those fights ) .
There, I hope it answered your question.