According to the wikipedia page, every group is indeed the automorphism group of some graph. This was proven independently in
de Groot, J. (1959), Groups represented by homeomorphism groupsGroups represented by homeomorphism groups, Mathematische Annalen 138
and
Sabidussi, Gert (1960), Graphs with given infinite groupGraphs with given infinite group, Monatshefte für Mathematik 64: 64–67.