First, since your auction is a mobile bidding auction, you should definitely let guests pre-register and therefore pre-check into the auction. Ideally, you want to know that they plan to attend and that you have a method of payment to make check out easier. Taking a cue from airlines, send out an email the night before the auction allowing guests to register and/or confirm their existing registration. Then, at the event, have a volunteer or two greet people and ask if they pre-registered. If so, they can keep walking and find that bar. This thins your crowd dramatically so there’s never a line.