As far as I can tell, there aren't many clubs around boulder, sadly, just a ton of race teams and a few weekly hammerfests that you nor I can participate in. The club I'm in is women only, and by your stats I'm guessing you are male.

Try BlueSky Velo in Longmont. They might have more to offer you in winter, when everyone is doing base miles. I think they are hammering right now.

There are some shop rides - Bicycle Village has one - but those can be sketch.

Good luck, and I hope you get some better replies here -- I'll be looking too.