Do you know about the Nashua River Rail Trail? It runs from Ayer to north to the NH border. It's probably about 15 miles from where you live to Ayer, and then about 12-13miles for the length of the trail, so that's in your window. I don't think it would be too crowded on fall weekends.
As far as "riding with other people", you can take a look at the Seacoast Century put on by the Granite State Wheelmen. It's Sept 20 & 21, there will be lots of riders, and you don't need to go the whole 100 miles (there are convenient ways to shorten the ride). I think registration ends soon though.