  We present a combined theoretical and experimental study of solid-state spin
decoherence in an electronic spin bath, focusing specifically on ensembles of
nitrogen vacancy (NV) color centers in diamond and the associated
substitutional nitrogen spin bath. We perform measurements of NV spin free
induction decay times $T_2^*$ and spin-echo coherence times $T_2$ in 25 diamond
samples with nitrogen concentrations [N] ranging from 0.01 to 300\,ppm. We
introduce a microscopic model and perform numerical simulations to
quantitatively explain the degradation of both $T_2^*$ and $T_2$ over four
orders of magnitude in [N]. Our results resolve a long-standing discrepancy
observed in NV $T_2$ experiments, enabling us to describe NV ensemble spin
coherence decay shapes as emerging consistently from the contribution of many
individual NV.
