Letters of invitation to promotions are very selective, and you're obviously sending them to people who are considered important.
There are some very simple, but important considerations with letters of invitation to promotions:
The example below is of an invitation to a high profile professional art critic to a special viewing of an exhibition at a new city art gallery. The art critic is an important person in the industry, read by a lot of high profile art collectors. The gallery is hoping to get some publicity, but taking the risk of the art critic's opinions. The art critic's staff have made arrangements for his attendance, so the letter is formally confirming arrangements.
The letter contains an actual invitation card, on special stationery, and a letter, which you'll note looks very much like a business letter, which, in effect, is what it is.
Dear… (title and name of person)
Exhibition of Modern Abstracts
We're delighted to extend an invitation to you for your attendance at our grand opening next month.
We've arranged for a special VIP viewing for you and your guests, with a cordon bleu dinner to follow and a question and answer session after dinner with the artists.
We will be pleased to provide off street parking for your media team, as requested. This parking will be available from 5AM, and one of our staff will be made available to liaise with your team throughout the day.
Your press officer has requested that no prior publicity of your attendance at the gallery be advertised. We confirm that no prior publication will be made.
Please contact me directly should you require any information or assistance regarding your attendance.