  We present the Global Rapid Advanced Network Devoted to the Multi-messenger
Addicts (GRANDMA). The network consists of 21 telescopes with both photometric
and spectroscopic facilities. They are connected together thanks to a dedicated
infrastructure. The network aims at coordinating the observations of large sky
position estimates of transient events to enhance their follow-up and reduce
the delay between the initial detection and the optical confirmation. The
GRANDMA program mainly focuses on follow-up of gravitational-wave alerts to
find and characterise the electromagnetic counterpart during the third
observational campaign of the Advanced LIGO and Advanced Virgo detectors. But
it allows for any follow-up of transient alerts involving neutrinos or
gamma-ray bursts, even with poor spatial localisation. We present the different
facilities, tools, and methods we developed for this network, and show its
efficiency using observations of LIGO/Virgo S190425z, a binary neutron star
merger candidate. We furthermore report on all GRANDMA follow-up observations
performed during the first six months of the LIGO-Virgo observational campaign,
and we derive constraints on the kilonova properties assuming that the events'
locations were imaged by our telescopes.
