Sometimes students ask for a letter of invitation for their family or friends. A common reason for visiting is graduation/commencement, but your friends and family are eligible to apply for a U.S. visitor visa at any point in time, with no specific invitation letter required. Pratt OIA does not write visitor invitation letters for friends and family, because it does not help their application.
Per the Department of State: Visa applicants must qualify based on their ties abroad/to their home country, rather than assurances from U.S. family and friends. A letter of invitation or Affidavit of Support is not needed to apply for a visitor visa. If you choose to bring a letter of invitation or Affidavit of Support to your interview, please remember it is not one of the factors used in determining whether to issue or deny the visa.
If you still want to provide an invitation letter, you can write an invitation letter yourself using this sample invitation letter template. If you want, you can also provide your visitor with copies of your immigration documents (I-20, I-94, visa, passport) and a print out of your unofficial transcript and course schedule. Again, this is all optional and does not impact the visitor visa application.
Your friends or family should follow the procedure for the B-1/B-2 visitor visa application at the local U.S. consulate/embassy at which they will apply.
As stated in the above text from the Department of State: Providing evidence or statement of a temporary visit is most important. See the INA Section 214(b) Visa Qualification and Immigrant Intent for tips on that from the Department of State.
If your family is eligible for ESTA/visa waiver, they should follow their particular passport rules for entering as visitor under the U.S. visa waiver program. The same temporary intent is required.