Yeah... That is a sticky situation, I agree with just cmoing right out and asking if the person you asked is able to make it giving the stress they have in their lives right now. It maybe that they can not afford it now or really should be going to get away from things. Put it out there that you have to know for sure or move on and their still more than welcome to come.
I am not having an MC, we're winging it, not a big group we're thinking 25 at the most, we have 16 booked.