Apologies for the really late reply, but I don’t think “marginal risk” in this context is well-defined. The marginal risk to yourself grows linearly in the number of people to first order. You could feel responsible for the marginal risk to all the other party goers, but they are people with their own agency, you aren’t (in my opinion) responsible for managing their risk.
If you’re deciding whether or not to add the (n+1)th person, what matters is the marginal risk of that decision.
Apologies for the really late reply, but I don’t think “marginal risk” in this context is well-defined. The marginal risk to yourself grows linearly in the number of people to first order. You could feel responsible for the marginal risk to all the other party goers, but they are people with their own agency, you aren’t (in my opinion) responsible for managing their risk.