Wedding Guests

I have guest who are inviting more people than were listed on the invitation. For instance, we invited my future father-in-law`s best friend and his wife. They RSVP`d with six people. The two of them, their son and daughter-in-law and two children 8 yrs and 6 yrs. Another instance we invited a close cousin and he invited his daughter and son-in-law and their two children. (These are people I don`t know on my fiance`s list). What is the proper way to keep my guest list from multiplying on itself? Can I have my fiance call his people and tell them that we just don`t have room for their additions? HELP!!! At this rate I could out grow my room and end up paying for this wedding through my 5th wedding anniversary.