Originally Posted by BigThor
Or Ultron could be Hank Pym's motivation for joining The Avengers because he feels responsible for Ultron's actions.
Very good point.
Although you could also say that SHIELD actively recruits him, even if he's not officially become (Gi)Ant-Man yet. We know that Pym was originally referenced in Thor
as an important colleague of Selvig, so he's probably already well-known as a genius scientist who could add significantly to the Avengers' "brain trust" of Tony Stark, Bruce Banner, and Erik Selvig.