The Big Game has become a popular night for gatherings so send out an email or electronic invite after the last playoff game has been played. Let your invitees know if they're welcome to bring guests (if they're not, then don't mention it and address requests on a case-by-case basic) and ask people to RSVP by the day before you plan to purchase your supplies. Then just assume there will be last-minute additions and dropouts and go with the flow!