Gaming Group Got Too Big

For the record, I realize this is an AWESOME problem to have, but I need help and advice nonetheless.

We've been hosting monthly for about a year. We started with a group of 8 and have slowly grown to about 17 regular players. Our house can really only accommodate 14 players. So far, at least two couples haven't been able to make it each time, so the numbers have worked out.

How would you handle invites so as to not over invite? In the rare event that everyone can show up we wouldn't be able to accommodate - I don't want to put myself or others in that awkward position if I can help it. 

My gut says to invite the regulars that have been playing the longest, then based on their RSVPs I can continue down the list of people to invite. I just don't know how to handle not including someone if it comes to that. :( Also we don't have space in the house to run two games at once.

Thoughts? Thank you!