Dedalus would have hovered around the Great Hall greeting and welcoming everyone, especially those he hasn't seen in person in a while. He gave hugs to Lily, Damocles, and I'm not going to finish this list because I'm sure he hugged everyone at some point.
During the discussion, he was attentive. He supported the ideas regarding non-violent protests and preventative measures. He would have expressed some regret about how his position at the Ministry was compromised by his quitting, as it would have been helpful to have someone else there undercover.