Pratt/OIA does not write visitor invitation letters for friends and family. If you want to have friends or family visit, you can write an invitation letter yourself. 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 required.
If you would like to provide an invitation letter, you can use this sample invitation letter template. You can also provide your visitor with copies of your immigration documents (I-20, I-94, visa, passport) and a copy of your student transcript or proof of enrollment from the Registrar.
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. Providing evidence or statement of a temporary visit is important. If your family is eligible for visa waiver, they should follow their particular passport rules for entering as visitor under the U.S. visa waiver program.