The Jewish Family & Children's Services runs free, drop-in New Moms groups (open to all faiths); none in Belmont but a few in neighboring towns. I used to go to the Arlington group (wow, 4 years ago!)
From their website:
http://www.jfcsboston.org/events_news/events_calendar.cfm
New Mothers Support Groups
Offered by Visiting Moms. Do you have a new baby? Would you like to meet other moms? New mothers are invited to come with their infants. Registration is not necessary as this group is free. For more information: Call : ###-###-#### for more information.
Arlington: Fridays, 10:00 to 11:30 am
Calvary United Methodist Church
300 Massachusetts Avenue
Brookline: Mondays, 11:00 am - 12:30 pm
Temple Ohabei Shalom
1187 Beacon Street
Cambridge: Wednesdays, 10:00 am to 11:30 am
Congregation Eitz Chayim
134-136 Magazine Street
Jamaica Plain: Tuesdays, 10:00 to 11:30 am
St. John’s Church
1 Roanoke Ave. (at Revere St.)
Medford: Mondays, 10:00 to 11:30 am
Medford Family Network (at Medford H.S.)
489 Winthrop Street
Newton: Mondays, 11:00 am to 12:30 pm
Temple Shalom
175 Temple Street
Somerville: Thursdays, 11:00 am - 12:30 pm
Somerville Family Network
42 Prescott Street (Cummings School)
Stoughton: Thursdays, 10:00 to 11:30 am
Striar Jewish Community Center
445 Central Street
Waltham (Spanish-Speaking Group): Tuesdays, 10:30 am -12 noon
Temple Beth Israel (behind Hannaford Supermarket)
25 Harvard Street
Watertown: Wednesdays, 9:00 to 10:30 am
Watertown Family Network
460 Main Street
... La Leche League offers breastfeeding support groups; the meetings I used to go to, there were moms with a range of babies-toddlers; they usually meet monthly, though:
http://www.llleus.org/state/Massachusetts.html
-- again none in Belmont, but Arlington/Cambridge, Lexington.