Welcome to Boulder! Are you female and do you have a road bike? If yes to both, Title Nine women's cycling club is having their open house ride on Sunday. Meet at the store downtown at 10:00, helmet & road bike required, no guys sorry.
Otherwise, the Rocky Mountain Cycling Club (http://www.rmccrides.com/index.html
) has rides all over the front range, their sunday ride is flatter, their saturday ride is hillier, you can join them for 3 rides before they ask you to join the club. I've seen people riding their fixies on the sunday ride.
My experience, having moved here a year and a half ago, is it's hard to find clubs in boulder that are sport/recreational rather than race oriented. Bicycle Village has a weekend morning ride (can't remember which day).