  We present a study of the metal-enriched cool halo gas traced by MgII
absorption around 228 galaxies at z~0.8-1.5 within 28 quasar fields from the
MUSE Analysis of Gas around Galaxies (MAGG) survey. We observe no significant
evolution in the MgII equivalent width versus impact parameter relation and in
the MgII covering fraction compared to surveys at z<~0.5. The stellar mass,
along with distance from galaxy centre, appears to be the dominant factor
influencing the MgII absorption around galaxies. With a sample that is 90%
complete down to a star formation rate of ~0.1 Msun/yr and up to impact
parameters ~250-350 kpc from quasars, we find that the majority
(67^{+12}_{-15}% or 14/21) of the MgII absorption systems are associated with
more than one galaxy. The complex distribution of metals in these richer
environments adds substantial scatter to previously-reported correlations.
Multiple galaxy associations show on average five times stronger absorption and
three times higher covering fraction within twice the virial radius than
isolated galaxies. The dependence of MgII absorption on galaxy properties
disfavours the scenario in which a widespread intra-group medium dominates the
observed absorption. This leaves instead gravitational interactions among group
members or hydrodynamic interactions of the galaxy haloes with the intra-group
medium as favoured mechanisms to explain the observed enhancement in the MgII
absorption strength and cross section in rich environments.
