When sending a bid request, does the bidder need to be a user in the system to respond?

No. The bidder does not need to be a user in the system to respond to a bid request. They will get an email notification letting them know they've been selected to bid on a certain billing item. They will click a link that will take them to an online form to enter the information and pricing. Once they're finished, they will submit their bid for your review.