The Idaho Whitewater Association is a group of almost specifically rafters that rarely represent the specific interests of kayakers.
If you are a kayaker, I would visit www.kayakidaho.com
and register and post in the forums there. Also as Ben said, try the Idaho Whitewater yahoo group. Also, attend the boat swaps in late April and early May.
But it's easy to make friends in Idaho. All you have to do is go up to Banks on a sunny weekend day and wait for people to show up to befriend. We have a large, humble, friendly kayaking community that is just waiting to become your new paddling buddy.
The town of Banks is north of Boise about 45 minutes. This is the put-in for the run you will want to start on, the Main Payette. It's also the take-out for a run you will want to do, the South Fork of the Payette (has a couple class 4s.)
I'm serious - if you can spend some time in Banks on the weekends or really anytime in the summer, you will have no problem making some great friendships and finding your crew of people to paddle with. Just hang out there, wait for boaters, and ask if you can tag along. Exchange contact information. And plan on being apart of maybe the greatest whitewater community anywhere.