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 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 per cent complete down to a star formation rate of ~0.1M_{sun}_/yr and up to impact parameters ~250-350kpc from quasars, we find that the majority (67^+12^_-15_ per cent 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 intragroup medium dominates the observed absorption. This leaves instead gravitational interactions among group members or hydrodynamic interactions of the galaxy haloes with the intragroup medium as favoured mechanisms to explain the observed enhancement in the MgII absorption strength and cross-section in rich environments.