Invitation letters are not required for your family or friends to apply for their visa. The following is directly from the U.S. State Department's website on visitor visas:
Note: Visa applicants must qualify on the basis of the applicant's residence and ties abroad, rather than assurances from U.S. family and friends. A letter of invitation or Affidavit of Support is not needed to apply for a nonimmigrant tourist visa. If you do choose to bring a letter of invitation or Affidavit of Support to your interview, please remember that it is not one of the factors that we use in determining whether to issue or deny a nonimmigrant tourist visa.
The most important part of the visitor visa application is the applicant's demonstration of ties to the home country, the short duration of the trip, planned return to the home country, and the ability to cover expenses. If you would like to write an invitation letter for your family or friends to come (for graduation, a visit, etc.) the letter should include: the name and relationship of the visitor to you, the length of the visit, who will provide the financial support for the visit, and the purpose of the visit. Your family and friends should bring proof of ties to their home country to their visa appointment as well, such as: copies of deeds to any property they own in their home country, bank statements of accounts they maintain in their home country, a letter from their employer stating they will return to their job, and any other documentation specifically requested or required by the Embassy or Consulate.
View sample letter
Travel Letters are not required at the border and we will no longer be writing them. If you plan on traveling, make sure you have a travel endorsement on your I-20 that is less than one year old (less than six months old if you are on OPT). It is also recommended that you provide evidence of your plan to return to the United States to finish your studies. This evidence can include: a copy of your schedule for the next semester, a copy of your tuition bill paid for the next semester, a letter from your employer stating that they expect you to return.
View the Visa Support Letter to renew your visa
Driver's License or State ID
The Pennsylvania Department of Motor Vehicles no longer requires a letter from the University to obtain a driver's license or state ID.
Social Security Ineligibility Letter
If you want to apply for a Driver's Licence or State ID but do not have a Social Security Number, you must request an ineligibility letter from the Social Security Administration. You do not need a letter from the University. The letter must be obtained before you go to the Department of Motor Vehicles (DMV) for your appointment. To get a denial letter you must present the following documents at the Social Security Administration (located at 717 State Street): I-20, passport, most recent I-94. When you go to the Social Security Administration, ask for a SSN rejection letter because you are an F-1 or J-1 student who is not employed on campus and you are seeking a driver's license. (Note: This letter does not prevent you from obtaining an SSN in the future.)