How to Write an Invitation Letter An invitation letter serves the purpose of inviting a guest to a party, event or celebration while conveying more information than a traditional invitation card. It serves two purposes : one is to invite the individual to the event, and the other one is to ensure that the person receiving the letter is going to attend. Business Invitation Letter An invitation letter is a formal way to invite peers and clients to events which are being hosted by the company. The professional invitation should be written in a formal tone. Next, in the body of the letter it is important to outline all of the information about the event. Friendly Invitation Letter A friendly invitation letter is similar to a business letter but contains less formal speech. Friendly invitation letters are used for a variety of reasons from engagement parties, baby showers and housewarming parties to wedding invitations.