Group airfare

Contact the sales office of the airline directly and ask for group sales. Remember someone may have to personally commit to all 16 tickets well in advance so be sure you have the money from everyone or someone will be holding the bag. And a travel agent may be of some help in this situation. Will not hurt to check with a travel agent specializing in group travel especially with hotels.

Price it out through Orbitz/Expedia/Kayak. Then look at a group travel specialist. Group travel does not necessarily result in cheaper air fare. The primary advantage of group travel specialist is they can block out a certain number of seats at a specific price, and you only have to put down a small deposit to hold them. You will have to make a group minimum (usually 10 seats) you don't have to submit the final name list until 90 days prior to departure. So if some people have to drop out, they're not really commited until 90 days out. Make sure everyone ponies up the initial deposit and make timely payments. The group travel specialist should have a group agreement with various airlines.
The downside of travel agents is they will definately try to up sell you on lodging, but at the same time they do get access to room blocks at hotels at less than advertised price.

We (parents, children, grandkids) used group ticketing on US Air to go to Brazil. We dealt directly with the airline group rep. The ticket price we eventually paid was the price on the day we made a $100/person deposit. We had to give names and passport numbers 90 days out, and made complete final payment 30 days (if I recall correctly) before departure. So you don't get a break in price, but you certainly do for payments. Other airlines may not have the same procedures. Suggest you look at prices and then call an airline or two. Good luck.

All the advice you've gotten is good. Here's another suggestion: Almost all airlines flying to Europe offer package vacation deals. These include airfare plus hotel and more if you want it. And since you're traveling in the fall, you can find some great rates that are all negotiable. Airlines guarantee the hotels a certain number of room nights. These are easily filled during the height of the tourist season but not so much during shoulder or off season. And if you have a group, you can negotiate even more. Granted, these will not be "Rick Steves" hotels. More likely larger ones but you choose the location. Additionally, at least in most cases with a package deal, your airline tickets can be changed without penalty. (Each airline is different.) I once did a week in London on a package deal with American. I stayed in a $300/night hotel for $125/night and my airfare was less than the published cheapest rate. (It was November so they were willing to give me a great price.) I even had to postpone at the last minute to the following week and it was all done without penalty.

No way would I good this through Orbitz/Kayak/Expedia. Might be useful for price guidelines, but the Seattle market is limited, and if you want good flights ( I.e. not U.S Air or weird stopovers) it is best to deal directly with the group sales office of the airline (if you know which one you want) or a travel agent. Elizabeth Holmes has a very good reputation in the Seattle area; I would call them. The trouble with Kayak etc. is they will show you some cheap options but they don't work. Recently I aw checking fares for someone and they showed some great fares on British Air for the route, just one stop in Heathrow. I gota excited for my friends as British is a good option from Seattle, but usually a higher price. Closer look, and it had long stopovers build in, making a 23+ hour flight. Ugh. Same route, with a same day flight on from Heathrow, cost much more. Who knows why. But a travel agent might be able to help you figure this out. For 16 people itis worth it to consult an expert.

Thank you for all the detailed advice. Much appreciated!

Another reason to not book through an online service such as Orbitz or Expedia is if you have a problem it will probably be difficult to get it straightened out. Whereas if you use the airline directly it will be easier to get the problem resolved since they are the point of purchase.