I too live in Longmont, and am a member of Poudre Paddlers. You can check out the website at www.poudrepaddlers.org
to see it they meet your needs. We have both whitewater canoeists and kayakers, and in addition to the Thursday night regular trips up the canyon, there are a lot of group trips throughout the season. Denver also has Rocky Mountain Canoe Club, and there may be others I'm not aware of.