From my experience the Montessori playgroup is a teacher run session with set activities, arts, crafts and interactive mat sessions. It's more like kindy. The kids don't 'have' to do any of them if they don't want to. The teachers encourage free choice/play so the kids can choose what activities interest them. Plus mum/dad are encouraged to spend one on one time with their child helping them with puzzles/activities. I was at an ordinary playgroup before I found Montessori and it was basically just like a mothers group meet up in a venue with art and craft available and toys but no structure as such.
I have found the Montessori playgroup to be really great for my little boy. He is becoming more confident in group situations, meeting lots of new friends and loves bringing home his art for daddy! He now asks to go to 'school'.
I can highly recommend it! I hope this helps. If I can help with any other information, please let me know.
Mum to a handsome little man