Christol St. and Clive St., Metuchen, NJ. 
Olmezer Park is a wide open playground very ideal for big playgroups. 
Basketball court in park, very spread out playground.  Friendly moms here, and the merry go round is a big hit for kids, but watch out for the little ones, since there is no speed control, bigger kids can spin it pretty fast. No bathrooms and a big sandbox.

For a local playgroup, you can visit

I met Alessandra Herzog from Better Homes Realty and she recommended some other nearby parks, hope I can get to them in the near future. thanks! If you need someone familiar with this area, visit her website