Is your family applying for a B-2 Tourist Visa?
Firstly, it is important that the family member(s) prove to U.S. visa officers that they have no intention to emigrate to the U.S., that their stay in the U.S. will be temporary.
If you would like to assist your family members, you may consider drafting a letter addressed to each individual formally inviting them to visit you. A letter of invitation is not required for family to make application for a B-2 Visa. If you will be providing financial support (food, lodging, etc.) for the visitor(s), provide them with a bank statement as supporting documentation.
For F-1 students, a template of the visitation letter can be found here.
For J-1 exchange visitors, a template of the visitation letter can be found here.
For more information on the B-2 Visitor Visa visit the U.S. Department of State website.