'By invitation only'

I am a little confused. You want to put 'by invitation only' on the invitation? It is assumed that if you are giving them an invitation, they are invited... no?

Unless you are hoping to stop people from inviting guests, etc to your wedding. In this case you write the name of the person or persons (Ms. Jane Smith or Ms. Jane Smith and Guest or Mr. & Mrs. Dan Dolt or Mr. & Mrs. Dan Dolt and Family) on the invite. If there is not 'and Guest' or 'and Family' than they SHOULD know that just they are invited. Most people understand that or will at least call before putting down an extra guest, etc...

If however, you are really worried about people ignoring or misunderstanding who is invited, you can always put on the reception card 'X (number) of spaces have been reserved in your honor'. If people cross out that number (HOLLA MJDOC) - then ... well then you probably shouldn't have invited them in the first place.