As has recently been shown experimentally, adatoms from group 14 exhibit distinct growth characteristics on Ru (0001) at submonolayer coverages. In particular, Pb and Sn atoms agglomerate in dense islands of the c(2 x 4) or c(2 x 8) symmetry with local coverage of 1/2 monolayer (ML), respectively, already for much lower deposition doses, whereas Ge atoms self-assemble in the dilute (root 21 x root 21) structure with adatom density of 1/7 ML. To trace the observed differences, we employed ab-initio density-functional-theory simulations and followed the early stages of growth of Pb, Sn, and Ge on Ru(0001) in a systematic manner by examining the potential energy surfaces for successive adsorption of up to several atoms of each element within a large surface unit cell. This way the effective lateral interactions between adatoms were recognized in the low-coverage regime, and their most favorable dimer and trimer configurations were identified. Interpreting them as initial building blocks of the adsorbate structure enabled us to clarify the tendency of various considered elements towards formation of a dense or dilute ordered overlayer on Ru(0001) at low nominal coverages.