Generating a complete graph for a given set of nodes is pretty trivial (and also fairly uncommon), so JUNG doesn't provide a method to do this. The code would look something like this:
for (V v : g.getVertices()) {
for (V w: g.getVertices()) {
g.addEdge(createEdge(), v, w);
}
}
(You'd need to supply an implementation of createEdge().)