They don't get paid until they can prove (usually with your paycheck) that you are "working", have completed "certain milestones" (as the Social Security brochure below words it). You get to choose to work with them and they get to choose to work with you so I would ask whichever network you decide on, the specifics of how it all works:

http://www.ssa.gov/pubs/EN-05-10065.pdf