In general I recommend sending a thank you note only to people with whom you made a significant connection.
The problem with that approach is that, in the group interview at MGH, everyone basically gets one or 2 questions that you answer in front of everyone. There's no chance to build a rapport and chat about stuff like you have with individual interviews.
On the upside, the group format is not nearly as frightening as it sounds before you get there, the day is over faster and you only have to answer "do you have any questions for me?" (and other generic questions) one time.
FWIW OP, I sent a TY note to the chair and emails to everyone else (and the chair as well).