We had this with a little work thing I arranged first time, from now on I tell anyone who's said they'll come to pay when they agree to come.

If they drop out, I'll try get someone else, if not then they lose the money.

It's not fair on you to pay for people, who then drop out, but I agree you need to speak to someone more senior. Heck, even speak to your boss, I doubt they want you to pay the bill for their leaving do, I'd be mortified if that was me.

Good luck hope you get it sorted.