  We present a theory of electron-mediated interaction between adatoms in
graphene. In the case of resonant scattering, relevant for hydrogentated
graphene, a long-range 1/r interaction is found. This interaction can be viewed
as a fermionic analog of the Casimir interaction, in which massless fermions
play the role of photons. The interaction is an attraction or a repulsion
depending on whether the adatoms reside on the same sublattice or on different
sublattices, with attraction dominating for adatoms randomly spread over both
sublattices. The attractive nature of these forces creates an instability under
which adatoms tend to aggregate.
