You can't really do anything to ensure they get visas other than provide an affidavit of support which might help but may not.  They need to be able to submit proof that they have no immigrant intent and only intend to visit for a short period of time and then will depart.  For that they need to present proof of their strong ties to their home country by showing evidence of jobs, business, property, school, etc.  If you say they are from Iraq, that is probably going to be rather difficult, to be honest,especially given that you are now a US citizen and the situation in Iraq.  Their intent will probably be presumed to be that of intending immigrants.