I take group classes for two weeks every summer (at SummerKeys) as well as private lessons daily. Our teachers learn ahead of time what people what to achieve and then tailor the group to the needs. Often they are a combination of technique, master class (most players are still fairly new, but advancing) and some group playing. That's different from the ensemble playing it sounds like you're doing. I started ensemble playing by sitting in on the youth orchestra in town. Being the old guy in the back, next to an 12-year old who could play rings around me, was both daunting and enriching. Keep looking for the group that meets your needs.

There are also some terrific three and four-part cello choir pieces to be had that are not terribly difficult. They get you beyond playing in unison.