We address the interaction of graphene with 3d transition-metal adatoms and the formation of localized magnetic moments by means of first-principles calculations. By comparing calculations within the generalized gradient approximation (GGA) to GGA + U we find that the electronic configuration and the adsorption geometries can be very sensitive to effects of local Coulomb interactions U in the transition-metal d orbitals. We find high-spin configurations being favorable for Cr and Mn adatoms independent of the functional. For Fe, Co, and Ni different electronic configurations are realized depending on the value of the local Coulomb interaction strength U. Chemical control over the spin of the adatoms by hydrogenation is demonstrated: NiH and CoH adsorbed to graphene exhibit spin S = 1/2 and S = 1, respectively.