You can write it stating that she can stay with you during her visit. However, those letters do not help much and are often totally ignored. She needs to apply for a B-2 tourist visa and must show that she has compelling reasons to return to her home...that is what the visa officer really wants to see. Since there is no U.S embassy in Iran , she will have to go through the embassy in Jordan or Syria.
Go on USCIS.gov website and see what they have to say about it. They also have a hotline where they can answer any of yoru questions. All the best!