Riding in a group is faster and more sociable, and drafting is the main reason it feels easier. It also comes with skills and etiquette that keep everyone safe.
Riding closely behind another rider can reduce the power needed at the same speed substantially, often quoted as roughly a quarter to a third or more, depending on speed, distance and group size
The benefit grows with speed, because air resistance rises sharply as you go faster
Smooth pedalling, predictable lines and pointing out hazards are basic group etiquette
Avoid sudden braking and overlapping wheels; most group crashes start with small contacts
A practical point for training: if a group ride is your hard day, treat it as one and keep the next day easy. Group rides often include unplanned surges well above threshold, which count as high-intensity work even if the average power looks moderate.