The best resources I've found have been FB groups and an e-mail list, along with connections from coworkers who also ride. One of our local bike shops has information about the FB groups and e-mail lists on their website, which is very helpful. We do not have clubs in the traditional sense, other than race teams, but clubs are often a useful way to find other riders.

After riding with a group for a while, things can get easier: I've found riders with similar goals and fitness levels to go with on other rides, usually arranged by text.

[MENTION=341575]RFEngineer[/MENTION] If you can get to Denver or Boulder, there are a ton of bicycling clubs, most of which have their own websites.