Hopefully your guests will have a proper understanding of the correct guest etiquette to know that this means that they are only to bring one guest. The truth is, however, that in this day and age, many people are NOT up on their etiquette. Our society no longer puts a priority on teaching the rules of etiquette, and so it is understandable that many people are prone to unintentional social blunders. Take special note of my use of the word unintentional. Be patient and kind with guests who err in this area – I can personally guarantee that most mean absolutely no harm, and therefore are deserving of your tolerance.
Should an invited guest return a response card indicating that they wish to bring more than one guest, you have the option of calling them to graciously and politely say, “I am so very sorry, but due to space limitations we are just not able to accommodate more than one guest per person. I do hope that you will understand”.
This is what is considered to be the ‘proper’ way of handling the situation you have described.