The United States requires visitors from a long list of countries to obtain a visitor visa in order to enter the country for pleasure. These visas may be easier to get if their issuance benefits a U.S. citizen. Writing an invitation letter, while not necessary, can be helpful in getting your friend from Nigeria approved for a visa. There is no form or regulation for these letters, but they should include certain information and be addressed to the U.S. embassy in Abuja, Nigeria.
Address the letter to the U.S. Embassy, Abuja, Nigeria. Just under the address you should state your name, your address inside the U.S. and declare that you are a citizen or permanent resident. If you are neither a citizen or permanent resident (holder of a green card) it will not be advantageous to write an invitation letter.
Continue the first paragraph by stating your occupation and perhaps your income. Proving your income may show the embassy that you can care for your visitor during your stay.
Continue by stating whom you are inviting, listing her name and address in Nigeria. Tell the embassy that you are inviting the person to the United States and end the first paragraph with this information.
State why you are inviting your Nigerian friend to the United States in the second paragraph. Remember that this is for a visitor's visa, which excludes having him enter the United States for work or to stay permanently. Perhaps he is visiting to see the country and enjoy the culture with you.
End the second paragraph by telling why it would benefit you as a U.S. citizen or permanent resident. Perhaps you enjoy her company, you are old friends that need to catch up or you want her to attend your graduation ceremony or wedding. The visa will be a priority if it benefits you as a citizen or permanent resident.
State whether or not the visitor will be staying with you at your address and whether or not you will assist him financially during his stay. The visa will not be issued if the embassy does not believe the person is capable of providing for himself during his stay. If you vouch for him he is more likely to be accepted.
End the last paragraph with details about the visitor's stay, including times and location for her entry and exit from the country. The more solid your plans seem, the more likely the visa will be issued.
Sign the letter after ending it with a respectful request for a visa. An example, "I respectfully request that you issue a visa so that (name of friend) may enter the United States as my guest."
Mail the letter to your friend in Nigeria so she may include it with her visa application at the U.S. embassy in Abuja.