Riding in a group, by which I mean, a properly ordered group, two abreast, and a minimal distance from the rider infront, is the way that Club cyclists ride when out on a social ride.
There is a good reason for this - at a normal club riding speed, say 15 or 16 mph, there is about a 20% reduction in effort, and considerably more at higher speeds, for anyone properly in the wake of another cyclist. For rider number 3, the benefit is even greater, up to 35%, but for the riders behind that, the benefit either remains constant, or deminishes due to other factors such as the concetina affects, all dependng on how skilled and disciplined the group is, as a whole, in group riding.
It is important to stay as close as possible to the rider in front, your front wheel should be no more than 12 inches from the rear wheel of the rider in front, to ensure that you are well inside the depression of air that is left in their wake. An experienced rider, when following another experienced rider, especially in a headwind, may cut that distance to only an inch, without EVER allowing the wheels to overlap. A great deal of experience (of both riders) is required when riding this close, and a good deal of concentration and anticipation also, not to mention the mutual trust, trust that the rider in front will not brake, or even momentarily stop pedalling. Such close riding is not recommended for the newcomer to group cycling, but is a skill well worth cultivating. It is called "wheel sucking" and scientifically that is a fairly accurate name for it too, because the rider behind, is riding in a "vacuum", albeit a partial one.
The video below, made by our friends at Cycling-Secrets.com shows graphically, that vacuum behind every cyclist. The size of that vacuum increases with the square of the speed (i.e double the speed, and the vacuum will be 4 times bigger), and is the major contributor of aerodynamic drag for any cyclist who is at the front of it. So once you plant yourself and your bike inside it, you not only make it 20-30% easier for yourself, you also make it up to 10% easier for the rider in front, by offloading much of the rearward suction that he/she would otherwise be subjected to. Group riding is truly gestalt when performed properly.
The airflow either side of that pocket of vacuum, is a stream of turbulent, eddying air, tumbling off the rider's shoulders and hips, and that is the other major cause of aerodynamic drag. Look at the video below and see how the "dirty" turbulent air tumbles off every surface of the rider and especially where there are changes in shape and section which creates an interference of flows. All that churning of air requires energy, and on the flat, there is only one place that energy is going to come from ultimately.
It is also important, for maximum aerodynamic benefit, to keep the same pace as the person along side you. Keep your handlebars in line with each other. It may not be too obvious or intuitive to see how this benefits you aerodynamically, but if you are on the front of the group into the wind, if you don't half-wheel, and don't accelerate ahead of the rider next to you, you will get an aerodynamic advantage. And for maximum aerodynamic benefit, you should close the gap between you and th eperson next to you. Watch the pros in the races, and see how they ride with their hands on the hoods, and their shoulders almost touching. This intimacy ensures that they are an aerodynamic unity. They have four shoulders, but only two turbulent eddying wakes. Again, the benefit increases with the square of the speed, with a theoretical benefit of up to 25% (but probably only about 10% in reality depending on how intimate you want to get).
If there is a sidewind component, then you will need to find that aerodynamic vacuum when drafting the wheel in front. A litttle offset positioning should find it, but it will never be as easy to get into, or as beneficial, as it is when the wind is directly from the front. This can lead to an echelon in extreme conditions.
Watch also for its sudden dissappearance on bends. The aerodynamic slipstream takes a straight line tangential path on corners, whilst you take a curved path. This would explain why, on cornering, you feel like you are sprinting to keep on the wheel of the bike in front, whilst the bike in front is actually keeping a good steady speed. Again, watch the pros in the races make good use of this to shake the limpets off their wheels.