  In this paper we identify objectively and analyze groups of galaxies in the
recently completed ESP survey. We find 231 groups above the number overdensity
threshold delta(rho)/rho=80 in the redshift range 5000<cz <60000 km/s. These
groups contain 1250 members, 40.5% of the 3085 ESP galaxies within the same
redshift range. The median velocity dispersion (corrected for measurement
errors and computed at the redshift of the group) is sigma_{ESP,median} = 194
km/s. We show that our result is reliable in spite of the particular geometry
of the ESP survey (two rows of tangent circular fields of radius 15 arcmin),
which causes most systems to be only partially surveyed. In general, we find
that the properties of ESP groups are consistent with those of groups in
shallower (and wider) catalogs (e.g. CfA2N and SSRS2). As in shallower
catalogs, ESP groups trace very well the geometry of the large scale structure.
Our results are of particular interest because the depth of the ESP survey
allows us to sample group properties over a large number of structures. We also
compare luminosity function and spectral properties of galaxies that are
members of groups with those of isolated galaxies. We find that galaxies in
groups have a brighter M* with respect to non--member galaxies; the slope
\alpha is the same, within the errors, in the two cases. We find that 34% of
ESP galaxies with detectable emission lines are members of groups. The fraction
of galaxies without detectable emission lines in groups is significantly
higher: 45%. More generally, we find a gradual decrease of the fraction of
emission line galaxies among members of systems of increasing richness. This
result confirms that the morphology-density relation found for clusters also
extends toward systems of lower density.
