This happens a lot, and caterers are used to it. They get additions and cancellations the day before the wedding.
So let's start with this, and then I'll guide you further. How far after the deadline have these people responded? Are there still a few weeks left before the wedding? Just let me know what the timing is here, and I'll advise. Caterers have contracts that might require you to pay a nominal fee for late additions...or they'll be totally fine with it, knowing it's just part of the package and that last-minute Yesses are great news for the bride and groom.
We'll figure that out in a sec...just let me know what the timeframe is here...
To clarify, we're dealing with this on a caterer's business angle. It would be bad etiquette for you to say to the late-responders that they can't come because they missed the deadline. This is one of those Taking the High Road times all wedding planners face from time to time.
Looking forward to hearing from you!
Sharon