Here are a few groups you can look at:

http://running.meetup.com/cities/us/ca/pleasanton/

ebtrailrunners.com

The meetup groups are a great place to meet fellow runners in your area.  Another thing you can do is go to your local running store and ask them.  Most running stores have groups that meet up, or they know of groups in their areas.

Hope this helps!

Happy Trail Running!

You are very welcome, let me know if one of them works out.  There may be other runners in your area that are looking for trail buddies.