I did invite my parents just to visit America, no arrival of new member. They got interviewed last week and got visa. I had sent the following documents:
1. Letter to consular requesting they grant visa to my parents (signed by me and my spouse with notary)
2. Employment verification letter (mine 25k/yr & spouse 35k/yr)
3. Our 6 months bank statement (had around 20k savings)
4. Copy of my PR and wife's US passport

Back home, my father included his bank statement (around 6 lakhs) although not required, he included anyway.

I didn't include:
1. Invitation to my parents( who will write formal invitation to their own parents)?, d
2. Tax returns (they are here for just visit, not as immigrants)