Since they are a group of people who are heavily into "personal responsibility," as you put it, then why not leverage that to make your point.
"I feel it is my personal responsibility to maintain the sanctity of our group and ensure the safety of everyone involved, especially those who are most vulnerable among us. It is important that we all understand the risks involved: should we decide to proceed, there is the possibility that our group will be directly responsible for knowingly facilitating the spread of a virus with a mortality rate estimated to be around 3%. Should this virus surface at our event, the likelihood of someone we all know succumbing to the infection is quite high.
Given the gravity of the situation and potential implications to our personal lives, are we certain that we wish to proceed?"
Good luck and stay safe.