For cyclists, the straight in line group is probably most efficient. The basic drag on an object is proportional to air density (less at high altitudes), proportional to "frontal area" (scrunch down low), and proportional to the square of the airspeed (v*v). Double the speed, and you have 4 times the drag. The other factor is a "drag coefficient" that depends on the shape of the object. You usually get the Cd from wind tunnel tests, but it's smaller if the object is "long" in proportion to its thickness.

If two riders are close enough so that the air doesn't "get between them" mother nature thinks they're one longer object, so they have a better "combined" Cd. The faster you go, the longer the gap can be, but it's often smaller than is comfortable unless you're alert and experienced.

At speeds below 10 mph or so, you'd have to be very close (and the effect would be small). Unfortunately, since things like stopping distance and maneuverability also go with the square of the speed (it's the kinetic energy that counts) the "allowable gap" for slipstreaming doesn't increase as fast as "space needed for feeling safe" does. You feel the "need" for increasingly more space as the speed goes up, and the increase in space that works doesn't increase quite as fast. Some skill and experience is needed.

When you follow a large vehicle, you're not really slipstreaming. You're just following a windbreak that reduces the local airspeed you're riding in. NO points for technical sophistication for this. It's just lazy - and usually dangerous.

When two riders align themselves to join the flow around their "connected" shapes, both riders get a reduction in Cd that makes it easier to get, and maintain, speed. As with formation flying or good sex, both participants must be willing and know what they are trying to do. Both must participate. (And they must trust each other not to crash and upset the flow.)

John