I am considering a small group tour to Macchu Picchu. I've traveled with Back Roads and Wilderness Travel, which are fantastic but pricey. I know a few people who have used OAT (Overseas Adventure Travel) and liked them. However, those travelers are a bit older and less energetic than I. Has anyone used OAT and some of the other companies mentioned (I've also heard Mountain Travel Sobek is very good) and can you tell me the differences between OAT and the more expensive companies? I like a lot of physical activity, and wide age ranges of travelers (one Backroads trip had people from 20's to 70's). Thanks.

What exactly is it you're trying to do? A day hike from Aguas Calientes up to Macchu Picchu, or 4-6 days of hiking beforehand? Are you wanting to camp out, or do you want to stay in lodges?

Check out Intrepid Travel, Imaginative Traveler, and G Adventures. All of these meet the criteria in your penultimate sentence.

I used Lost World Adventures. We had a gude and a driver. The " group" was three friends and me. The guide was excellent, the price was reasonable and the experience was unforgettable.
http://www.lostworldadventures.com/countries/peru/

Machu Picchu experience couldn't have been better, small groups allowing you to walk at your own pace, knowledgable guides, incredible food and organisation by the porters.
Even sleeping in the tents was comfortable plus what's better than being woken up with a mug of warm coca tea!
We left Machu Picchu feeling like we'd missed nothing. Cheaper than other official inca trails but this reflects in no way upon the quality of the tour. Would highly recommend booking with Tierras Vivas