There is a place in Arlington called ready, set kids that does some groups. I did one there when it was Isis and it was great! Also, there are a bunch of baby cafés for nursing moms that are free in the area and a ton of groups on meetup.com.
Jewish family and Children's services has great new mom groups. I went to the one in Arlington for my daughter's entire first year and made some really great friends.
You absolutely do not need to be Jewish to attend they are completely non denominational.
Re: Medford area mom groups