Here in Chicago there are a lot of cycling groups, clubs, teams, etc. However, members of the chicago cycling club (CCC) formed an on-line listserv sort of thing through Multiply.com. The members of the multiply list are members of the CCC, or friends of members, etc (it just grows by word of mouth). The purpose of the multiply list was to be able to meet up for more informal rides not officially sanctioned by CCC. It has worked really well. Anyone who wants to ride in a group posts his/her tentative rides plans a few days in advance, sets a starting location/time, and people can post their intentions to join (or make suggestions on distance, route, pace. etc). This way, we have been able to get regular group rides going even well into the winter months when the local club rides are few and far between.