Flying into one city and out of another

What you are trying to do is known as an "Open Jaw" flight: arrive in a city and come back home from another. It's called that way because if you plot the itinerary on a map it looks like...an open jaw. You don't need any agent. Just go to kayak.com and the "multi city" option. In the first line Enter from Omaha (or wherever you depart from) to LOndon LHR. In the second line enter from Rome FCO to Omaha. Search. Look at the option that come up, which one you like, purchase.
You can do the same in other website as well. A group means 10 or more, at least for most airlines.

Virtually any of the flight search sites will let you schedule that type of flight. Just choose the multi-city option. I like Kayak, but the others also can handle it. I use the sites to decide what route/price I want, although I often book directly with the airline, rather than through the search site (and sometimes get a slightly better price). For your return, you're more likely to get a direct flight to the US out of Rome, rather than Florence. I've flown out of Florence several times and have always had to transfer at some hub in Europe; also, departure times from Florence are often obscenely early because of the need to make morning connections in Amsterdam, Paris, Munich, etc. For travel within Europe, Skyscanner is a good website that includes the European discount airlines. For many routes, it's cheaper to fly than take a train. Just be aware that discount airlines often use outlying airports; pay attention that to the location, and factor in time lost and added costs for ground transfer to your destination. Also, CAREFULLY read the rules of hte discount carrier re luggage weight/number of carryons allowed, printing ticket in advnance, arrival times, etc; deviations from the rules are expensive. Finally, if the one-way ticket price seems high, ask about round-trip price; a couple of times it was cheaper to buy a round trip ticket for a one-way trip. I'm not sure what you mean by your question about a group. Looking for discounts on flights? I've not seen that, but you could always check with a travel agent. Identify the best price you can find, and see if a travel agent could do better for the group.

Ragan: Flying open jaw'd is almost the norm for people touring Europe. Often the cost is the same as flying into one city, and the savings is not having to back track. Your travel thru different cities can be more in a straight line, which is more efficient. Obviously you're in the starting phase of planning your trip, and planning is now so much easier with the internet at hand. When I'm planning a trip, I tend to look at websites like this one, and Rick Steves' Graffiti Wall for sections like Heroic B&B Friendliness looking for the same names to pop up time and time again.
We did your itinerary last Easter flying into London for a few days, and taking an EasyJet budget flight into Pisa. Then, we toured Florence, Chianti and flew out of Rome. Italy is almost worth a trip to itself, as there's just so much to see.

Thank you all so much for the words of wisdom! And for the tips regarding flying while in Europe! We did think about buying a round trip but only using one way but were wondering if that will put us on some kind of list for not using half of a ticket? If I find anything fun out I will keep you all posted!

Do not consider not using the other half of the ticket as your return segment probably will not allow you to fly.

I don't understand Gail's comment. There is little risk if you chose not to use the second half of a RT ticket. In theory, the airline MAY have reserved the right to charge you at the one way rate if you fail to use the return portion. However, no one has every reported that happening. If you made a regular practice of skipping the return portion, maybe you could get on some list but that probability is so low I would not be concerned. But use an open jaw ticket. Nearly all of our trips are open jaw and I never found it to be more expensive and often cheaper. Two trips back was into Rome and home from Zurich. It was $200/person cheaper than RT Rome.

There is no need to do anything like throwing away half of a round-trip ticket. The budget European airlines price tickets one-way and are very reasonable. You can easily book this: 1. Book your main flights into London and out of Rome. 2. From London to Paris, use the Eurostar train. Book way in advance (I think it is 120 days) and you will get a good price. 3. From Paris to Italy, assuming you have no interest in taking several days and stopping in southern France or Switzerland, book a flight on EasyJet or one of the others from Paris to Florence or Rome or wherever you like. (Pisa is a larger and better-served airport than florence, and would be good if you are going to Cinque Terre).

Frank. Is right, I did not mean you can not fly, I meant you would may have to pay extra fees, had not had my third cup of coffee when I posted, so sorry.