These days you dont need sponsor letter. They need welcome letter from a person already in US. The visa status of the welcoming person can be anything I guess. I am pretty sure that they dont need sponsor now. When I came I had to pay a lot of money. This was a sort of business then....its good now. The letter that u need to write is very simple. Stuff that u know him/her,,,,then u would welcome him/her, and then relation,,,friend /family etc......that should be enough. U should show ur willingness to help them settle in your area..... hope this helps