There are the Charles River Wheelmen, mostly riding in and around Boston, and the Seven Hills Wheelmen (more out toward Worcester). I just have gone on a few rides with the Nashoba Valley Pedalers who cover the 495 corridor - I plan to become a member sometime in the near future since I am close by and hope to do a lot more rides with them now.

The NVP for sure has rides suitable to beginners - Wednesday night rides are generally shorter distances and easier terrain. Tuesdays are their harder rides, usually mixed terrain, faster speeds, longer distance but even those often have a shorter/easier option. Thursdays are a constant course by which you can measure your improvement week to week. I've only ridden with them twice so far but they seem to be a pretty good and diverse group.

I haven't done anything with either Wheelmen clubs but I am sure they have similar intro type rides.

I am certain you can find a club! Keep us posted.