If you are a U.S. citizen or lawful permanent resident, and you have a friend or relative who seeks to come to the U.S. as a tourist (on a B-2 visitor visa), you might be able to help the person by providing a letter of invitation. Whether the person will be visiting a U.S. consulate and applying for the visa or already has a visa, but will still need to get past U.S. port or border guards, it can help smooth the way.
It's important to realize, however, that such a letter is not a requirement, and might as easily be disregarded by the U.S. officials considering whether to grant your friend U.S. entry. Ultimately, that person will need to convince U.S. officials that the visit to the U.S. will not be a permanent one, and that the other visa eligibility criteria are met.
Getting a U.S. tourist visa can be challenging, because the U.S. government is worried about the number of people who use these as a way to enter the U.S. and then never leave, or who overstay by days, months, or years.
The aim of your invitation letter would be to help show the U.S. consular officer who considers the visa application that the person has a specific plan regarding visiting (and eventually leaving) the U.S., perhaps has a place to stay, and will not need to find work in order to support him- or herself while here (if, that is, you plan to offer financial backup or support). Also see Reasons a U.S. Tourist Visa (B-2) Could Be Denied.
This should be a personal document, not something like a lawyer would write, so don't stress over making it sound official. Make sure to include both your and the recipient's name and full address, for example using the format shown below.
Also be sure to cover:
Be as specific and detailed as possible. Below is a sample letter for you to use as a guide.
You can give this letter to your friend or relative to take along to the U.S. consulate when her or she presents the various forms and documents that comprise an application for a U.S. visitors' visa and meets with a U.S. consular official.
If you plan to provide financial support to the visitor while he or she is in the U.S., and don't feel like writing a letter, you can fill out USCIS Form I-134, called an Affidavit of Support. Or, you can provide both a letter and a Form I-134.