A good place to start is a consulate where they are planning to apply for their B1s - they normally have a full list of documents required.
Consular policies do vary, but if their official reason of visit is to see you, they will need a proof of your legal presence in the US (copy H1-B stamp & I-94, bith sides ). If you will be supporting them for the lengh of their trip, you will also need to submit the affidavid of support.
You can also read more at http://travel.state.gov/visa/tempvis..._visitor3.html
PS Not a lawyer, so not a legal advice