  We study the formation and the evolution of galaxy groups in a critical
universe, showing the importance of secondary infall for their dynamical
evolution. Merging is only slightly accelerated if galaxies have massive halos,
because the mass initially associated to the individual galaxies is soon
tidally stripped. Stripping is particularly effective for infalling galaxies,
which thus avoid merging. We find that, as a rule, merging is effectively
terminated when infall becomes dominant. We look for compact groups in our
ensemble of simulations, and compare their statistical properties with
Hickson's compact groups. We then discuss in terms of the Press & Schechter
formalism the statistics of such groups and their evolution in different
cosmological scenarios. We show that compact group formation is an ongoing and
frequent process in a critical universe. Our model reconciles the contradiction
between the observed absence of young merger remnants and the high rate of
galaxy interactions expected in compact groups. In open universes, instead,
earlier formation of groups and suppression of secondary infall makes it more
unlikely that compact groups survive as such until the present. We conclude
that the existence of dense and dynamically young groups of galaxies like HCGs
points towards a high-density universe.
