I totally agree with Missliz (!!) about spin classes NOT being the place to meet riders.
Althought I mbike exclusively, I think the principal is the same. I've been on a mission for a few years to meet more biking women, as originally I only knew one other. MY solution- talk about riding to everyone I know. Eventually you talk to enough people and you start having conversations that involve "call me" and "hey let's do it". After a few calls you find out who is serious or not; who gets to stay on the "usual suspects will-call list".
You have to be patient.
After three years I finally have a really good list of weekday riders. Not everyone can come out all the time, but it's a fun bunch and we've picked up a few beginners this year too.
What about starting a casual club? We have a women's bike club here in addition to a regular road club. It was started by a few women who were originally a 24 race team, who decided to spread the gospel. It's grown from about 20 member to 75 in a year. I found out about it by a flyer at the bike shop.
Missliz, sounds like you have nice club too. I've been real disappointed in mine for a number of reasons but that's a whole nother thread.
Irulan
2015 Liv Intrigue 2
Pro Mongoose Titanium Singlespeed
2012 Trek Madone 4.6 Compact SRAM