The weak ferromagnetism reported for graphite and related carbon nanostructures is frequently related to the magnetic coupling of point defects such as vacancies or hydrogen adatoms that interact with only one sublattice in the bipartite graphene lattice. In this paper, using density functional theory calculations we study the magnetic coupling between point defects, such as carbon adatoms, which form bonds with atoms located on both sublattices. We show that there is ferromagnetic coupling for small separations between the adatoms. Further, we demonstrate that it is energetically favorable for C adatoms to agglomerate. Our results indicate that the magnetism induced in graphite by irradiation can be explained as being due to the existence of adatom agglomerations and small graphene flakes formed from isolated adatoms rather than uniformly distributed point defects.