My favorite way to extend an invitation for a dinner party, especially one of that size, is to call. The telephone is the perfect way to reach out and ask someone to join you. Once they accept your invitation, you can ask another couple who would mix well with the group you are forming and so on and so on. You can customize your guest list and put together a dinner party with chemistry this way.
It is important, I believe, to follow up with a Pour Memoir, or To Remind, card. You can either find a lovely blank card and ask your calligrapher to write it out for you or you can have some printed with your names and spaces to fill in.
The verbiage should go like this: