I know the shuttles from SFO (which provide door to door) service are usually $15/person. I think OAK shuttles run about the same.
I assume they are checking into a hotel or staying with someone in downtown SF? If so, I think it's worth the extra cost for the shuttle. It's door to door. Easier to deal with luggage (which sounds like they have a lot). You also don't have to deal with the transfers to BART or figuring out how to get from BART to the hotel.
I also will second the comment about the BART ticket machines being very confusing. Even after having lived in SF, when I go back I still get flustered.
Take an airport shuttle.
http://www.flyoakland.com/shuttles.shtml
[This message has been edited by StudentExplorer (edited Mar 18, 2004).]