Riders get registered, not the bikes.... so, unless the event offers a tandem, family, or group rate you can pretty-well assume the expectation is that each person on a tandem will register.
Large rides with a wide variety of different types of riders demand a lot of attention and common sense from a tandem team. Every team is a bit different in how they approach these types of events and you'll need to figure out what works best for you, taking into account how strong your team rides, how well developed your tandem-handling skills are, and what constitutes a "good time" for you on a tandem.
Some tandem teams we know actually like to start out near the back and do the 'on your left thing' through the pack... but I'll be darned if I know why. Others are strong enough to ride with the lead group or "B" group who tend to have pretty good bike handling skills and fewer squirrels than slower groups. Still others like to start early to get out in front of the masses and hang with the lead teams for a while when they come through.
Just keep your wits about you, stay well off the rear wheel of any other bikes, and be mindful of kling-ons / wheel suckers.