actually depending on the type of playgroup you want your kid to join, parents will recommend different schools.
For eg if you are looking for a playgroup for your child to be academically ahead, i m sure there will many that will recommend Growing up Gifted or (i forgot the other one).
If you are going for a more musical one, there will be Kindermusik.
If you want your child just to be go and make friends and socialise, child care centers or Apple tree would be good and others.
Depending on you, which age do you wany your child to go for playgroup? 18 mths? 2? 3? I think you should register months ahead if you confirm that you want a place in the school.Especially if its those popular ones.