  We aim at defining homogeneous selection criteria of small galaxy systems in
order to build catalogues suitable to compare main properties of pairs,
triplets, and groups with four or more members. To this end we use
spectroscopic and photometric SDSS data to identify systems with a low number
of members. We study global properties of these systems and the properties of
their member galaxies finding that galaxies in groups are systematically redder
and with lower star formation activity indicators than galaxies in pairs which
have a higher fraction of star forming galaxies. Triplet galaxies present
intermediate trends between pairs and groups. We also find an enhancement of
star formation activity for galaxies in small systems with companions closer
than 100kpc, irrespective the number of members. We have tested these analysis
on SDSS mock catalogues derived from the Millennium simulation, finding as
conservative thresholds 76% completeness and a contamination of 23% in small
galaxy systems, when considering an extreme case of incompleteness due to fiber
collisions. Nevertheless, we also found that the results obtained are not
likely affected by projection effects. Our studies suggest that an extra galaxy
in a system modify the properties of the member galaxies. In pairs,
galaxy-galaxy interactions increases gas density and trigger starbursts.
However, repeated interactions in triplets and groups can generate gas
stripping, turbulence and shocks quenching the star formation in these systems.
