To write an invitation letter for a U.S. visa, be sure to include the reason for the visit, where and how long the visitor is planning on staying and information about travel payments. Also include information on expenses.
The goal of the letter is to show the consular officer that the person visiting the United States has a specific plan regarding the visit, including a specific time to leave. A United States citizen must write this letter before giving it to a relative or friend to take to the consulate when presenting an application for a visitors' visa. This is a personal document, and it is best to keep it friendly and informal, not like something a lawyer might write.
When writing the letter, include a few of the places the person may visit and how long the visitor plans on staying in certain accommodations. It is also important to mention if there is a plan to cover the transportation costs of the person and if there are plans to pay for the person's expenses while he is in the country. It is important to be as specific as possible when writing this letter, and samples are available online to help.