If it isn't too late, I'd suggest just replacing the joist. If the decking isn't down yet, it should be fairly easy to remove.
If it is too late for some reason you can do one of two things...
1) Like you said, add another joist next to the warped one using a lot of nails to secure it. You could get away with screwing the decking into the warped joist I imagine.
2) You can essentially shim the low spot on the warped joist by attaching a ripped tapered shim from a piece of treated wood, reasonably matching the low spot on the joist. The shim would attach to the top of the joist.