Learn the double paceline and rotation: ride two abreast, hand over the lead cleanly, save up to 159 W by drafting and know when it is legal.
The honest answer: the double paceline is the group formation in which everyone alternately shares drafting and lead work. It only works if everyone rides calmly, evenly and with calls. The gain is measurable: in a field study with 13 competitive riders, the power needed by the rider in the draft dropped by 111 to 159 watts, depending on the leader [3]. The drag coefficient dropped by 36 to 50 percent [3]. That is an advantage you do not have to buy, only share.
The effect is clear at higher speed too. In field measurements of the US pursuit team at 57 to 60 kilometres per hour, the second rider in the line needed only 430 watts, the leader 607 watts, about 71 percent [4]. That was on the track at very high speed, so the finding is a transfer to the road; the order of magnitude (the second rider saves a lot) remains.
The group does not win through hardness but through evenness.
The topic has two sides: technique and rules. Without agreement, a double paceline on the road quickly becomes dangerous. Realistically you need a few group rides until the routines sit (experience value). Prerequisites are drafting and hand signals.
Ride two side by side, both lines at the same speed, with about one bike length to the front (practice value, larger for beginners). The lateral distance is enough that the bars do not touch.
The speed of the front sets the group. Whoever is in front (and that applies to every change alike) holds it constant and avoids needless accelerating or braking. In a group even pedalling counts more than power.
Milestone (practice): 5 minutes of two-abreast without the gaps stretching like an accordion.
The leader gives a clear signal (for example arm movement or the call "change") and then pulls out cleanly to the side. The group should agree beforehand who lines up where.
The relieved rider rolls back along the side of the line and rejoins at the end without braking or cutting the line. The next riders move up.
Change at regular, short intervals so nobody makes the wind for too long. If you are tired, say so and stay at the back. A rider who tortures himself beyond his limit breaks up the group faster than one who honestly takes a few minutes off.
Milestone (practice): 5 changes in a row without anyone letting the pace drop noticeably.
On public roads, bicycles may be ridden side by side if this does not obstruct traffic; otherwise single file is required [1]. More than 15 cyclists may form a closed formation and then ride two abreast on the carriageway, provided the formation is clearly recognisable to others as such [2]. So a smaller group is bound by the rule not to obstruct.
Groups are the risk: of 128 head impacts examined in professional cycling, 88 percent happened in road races and predominantly in collisions involving several riders [5]. So the group raises the risk. Keep your distance, announce changes, do not touch wheels. This guide explains and motivates, it does not replace supervised group training.
Agree on a simple signal system and a target before every ride: "change after 30 seconds" or "each one minute". When everyone knows when it is their turn, the changes run by themselves. That matters more than any technique: a small plan takes the stress out of the change.
Two riders side by side, both lines at the same speed, with a gap to the front. The leaders peel off to the side on a call, roll back on the outside and rejoin at the end. This is practice standard; details like which side you agree beforehand in the group.
In a field study the power needed in the draft dropped by 111 to 159 watts depending on the leader [3]. In track team riding the second rider was at about 71 percent of the leader's power [4]. The value depends on the leader and the speed.
Bicycles may be ridden side by side if this does not obstruct traffic [1]. A closed formation of more than 15 cyclists may ride two abreast [2]. You check the legal position for your individual case locally.
Reflex braking can make the rider behind run into you. Stay calm, move out to the side, keep your distance and call out the hazard. This is practice knowledge. How fast the crash spreads depends on speed and gap.
The Belgian Tourniquet: Share the Lead Work in a Group Evenly