Party Etiquitte - Loose Lipped Guests

We are having a party billed as a "private" party. Due to the small size of our place, there were some friends who we could not invite. Now a friend (who lives a bit further out of town, and we haven`t been in regular touch with) has apparently heard about the party and sent an email asking if we "are having a party without them". We presently well over capacity for the party (with the RSVPed guests). What is the appropriate response? Ignore the message? Explain the situation to them? Invite them?

Thanks in advance,

J&E