If it really is an "organized" ride for thousands of riders of all skill levels, you don't really need to know/do anything special other than to be reasonably prepared for the time/distance of the ride, bring along water bottles, and bring money. And maybe sun lotion on a sunny day, or rain gear on a threatening day.
But not all "organized" rides are that, and you need to be a little prepared for at least minor chaos -- having rest stops run out of water/food, maps or cue sheets that aren't especially accurate, etc. More money, an extra water bottle, and a cell phone will get you through most of these.
Otherwise you need to look out for others in the ride, to protect both them and you. If you've not ridden with a group before you should observe other, more skilled riders to learn a little bicycle group etiquette, and you particularly need to be conscious of the safety issues involved, such as riding too close together, riding 3-4 abreast in traffic, making sudden turns and stops without being aware of the riders around you, etc.
With that many cyclists there will be some idiots. Don't ride like an idiot just because a bunch of other riders are -- do your best to set a good example. And with that many bikes there will be some folks driving cars who will get frustrated and testy -- understand that they are NOT enjoying themselves motoring along at 5mph watching you guys clown around. Show some consideration.
Oh, and bring some toilet paper, or (my preference) a small packet of "baby wipes".