According to Wikipedia, Brooklyn Decker gave birth in September 2015 and Grace and Frankie's second season was filmed July through November 2015.
June Diane Raphael is apparently pregnant now during the third season shoot (she said on one interview "No I'm not" but it appears to have been a joke, because she said she was and discussed it at length in another interview). She also had an infant on set with her during the first season - she said in an earlier interview that she auditioned three weeks after giving birth because she had to get out of the house. She's grateful that Jane Fonda, a feminist activist, made sure it worked to have a child on set.