NORTHAMPTON — To at least half of the 10 people gathered for Fritha Pengelly’s dance class at Thornes Marketplace in Northampton on a recent Tuesday afternoon, just getting there was a challenge. Some of them have trouble controlling their limbs. Others have an unsteady gait or a difficult time with their balance. Yet here they were seated in a circle following a synchronized sequence of complicated body motions. Pengelly had them raising an arm and curling it around the back of their heads while she tapped a steady beat.