There are occasional group rides of a group of folks from the Seven Hills Wheelmen, but nothing regular. There is good stuff there, but it's a matter of finding it then figuring out how to link up the single track bits with the fire roads.

A good start is to find the trail that goes around muddy pond (http://goo.gl/maps/lzQw5) off of the rail trail - it's a great easy flowy trail. After you've done that, you're in a good spot to look for more single track on the other side of the rail trail - it's a bit of a maze, but after a couple of rides, you should be able to figure it out. Some of the trails are officially horse trails, but are well suited for bikes and don't see much horse traffic.

The maps linked below are useful for finding your way around, but don't actually show much of the singletrack:

http://www.mass.gov/dcr/watersupply/.../wracrbike.pdf
http://www.mass.gov/dcr/watersupply/.../wrachorse.pdf