  An obstacle for spin-based quantum sensors is magnetic noise due to proximal
spins. However, such a spin cluster surrounding the sensor can become an asset,
if it can be controlled. Here, we polarize and readout a cluster of three
nitrogen (N) electron spins coupled to a single nitrogen-vacancy (NV) spin in
diamond. We further achieve sub-nm localization of the cluster spins. Finally,
we demonstrate coherent spin exchange between the species by simultaneous
dressing of the NV and the N states. These results establish the feasibility of
environment-assisted sensing and quantum simulations with diamond spins.
