A grouping method and unit (6) for forming a line (7) of products (2) on a packing machine (1); a first feed conveyor (11) feeds a succession of spaced products (2) along a straight, horizontal grouping path (P) at a first speed (VI); a first compacting conveyor (20) moves a first compacting member (23) along the grouping path (P), downstream from the first feed conveyor (11) and ahead of a first product (2a) in the line (7), at a second speed (V2) slower than the first speed (VI), so as to slow down the first product (2a) and compact the intermediate products (2b) in the line (7) against the first product (2a); and a second compacting conveyor (21) moves a second compacting member (25) along the grouping path (P), downstream from the first feed conveyor (11) and behind a last product (2c) in the line (7), at a third speed (V3) higher than the first speed (VI), so as to compact the last product (2c) against the intermediate products (2b) in the line (7).