Realizing groups as automorphism groups of graphs.

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 groups, Mathematische Annalen 138

and

Sabidussi, Gert (1960), Graphs with given infinite group, Monatshefte für Mathematik 64: 64–67.

In the topological setting or if you want to relate the size of the graph to the size of the group, there are two relevant results:

(1) Any closed subgroup of $S_\infty$, i.e., of the group of all (not just finitary) permutations of $\mathbb N$, is topologically isomorphic to the automorphism group of a countable graph.

(2) The abstract group of increasing homeomorphisms of $\mathbb R$, ${\rm Homeo}_+(\mathbb R)$, has no non-trivial actions on a set of size $<2^{\aleph_0}$. So in particular, it cannot be represented as the automorphism group of a graph with less than continuum many vertices.