Even though the details are different, I am sort-of in a similar situation. Some colleagues have a higher need to chat, and they included me in their chat groups in social network(s).
Exactly as you explain there is a conflict between their needs and my needs.
However, I clearly understand that they did not add me because they hate me or because they expect ...
Two things stand out to me:
You did not ask to be part of this group.
What others think is none of your concern.
Those two things being the case, you have no obligation to participate in the group. However, I understand you work with key individuals of this group, i.e. the new member of your research group. You don't want to alienate that person.
I suggest ...
I cannot judge on whether the committee members’ comments on your attitude are justified as I was not present at the event; and much of people’s impressions on one’s attitude often have to do with the way, the facial expressions and the demeanour in which one spoke.
The reason why the adjudicators regarded your remarks that you did all the work alone as ...