Sometimes it seems like finding a new friend is harder than finding a significant other! I think as we get older, it's also harder because women are so busy with their kids/families, etc.
Meetup.com is a good suggestion, look for a book club, or women's social group. Also, maybe before your group rides begin, ask if anyone would like to go out for coffee/drinks/lunch, etc afterwards? Maybe there are some other newbies who are looking for friends