We did Music Together from age 1 to 2.5. The class was mixed-age, but the vast majority of the kids were toddlers, with an occasional baby brother or sister or older preschooler attending with a younger toddler-age sibling. We did develop friendships with a few families in the class - our location was at a church with a small playground outside, and several members of the class stayed to play after the class. My child is 4.5 now and we still listen to the CDs in the car from time to time. If we'd stayed in the area, I am sure some of the friendships would have remained active - as it is, we moved about an hour away when my child was 2.5, but I still keep in touch with some of the parents I became friends with through Music Together and we still get together occasionally.
The class had lots of parent participation and was pretty active - dancing or marching for some songs, some lap songs, playing with scarves and bells, and lots of exploration of percussive instruments.