An invitation letter is a formal request of someone's company. If inviting multiple people, it is best to have a template printed on to several cards, or in your own word document to be printed out and sent by mail, or delivered electronically.
Ideally, it should have your own name in the first line with the words 'requests the pleasure of your company', with the names of the recipient(s) either above or below it. Full details of the anniversary celebrations, including location and date, should then be written. Any costume details - black tie, cocktail dress, lounge suit and so on - is detailed, as well as the reception time and 'carriages', meaning the time the party celebrations are to break up and end.
Also say whether guests are invited as well, usually in the space where the recipients' names are, perhaps with 'and family' attached. Choir anniversaries, either for church or social choirs, are times of celebration of the human spirit, and usually a boisterous occasion with several like-minded souls all meeting up. The benefits of singing in a choir, as well as making new friends, are spiritual and health-related in equal measure.
Singing uses the entire capacity of the lungs, and helps gas exchange due to the necessity of getting oxygen into the organs. Choirs sing a variety of music, both secular and religious, and clever choir directors mix the two ingeniously when programming concerts and recitals to showcase the harmonies and solo singing of members. Parties should be essential.