I think there are technically 3.
Snake River brewers meet monthly (in Nampa or Meridian I think). They're a big active group with lots of attention to bylaws and such. I'm in the
Boise Brewers (aka Brew Con Brew Club) and we meet monthly in Boise (usually on the last Thursday of the month.) We usually have 8-12 people, bring some of our beers to share and critique, and get together for group brews periodically. We're
meeting Thursday night at Sockeye at 7pm. I'm bringing a couple beers to share.
Idaho Brewing Association seems to focus mostly on group brews. They're brewing this weekend.