Check out Meetup.com. There are a ton of cycling groups on there that host group rides at every level from beginner to advanced, to rides that are more focused on the social aspect to hard core types. This group covers a wide range and is well run. There is also the WABA group that focuses on women and hosts group rides as well. Good luck!
Just start striking up conversations with others riding the same direction your riding. They are more likely to have a similar routine which makes future rides easier to join. Many cyclist are creatures of habit. Everyday I see the same people at the same place and at the same time. Once you notice the patterns, it’s easy to build connections that make riding even more fun and motivating