An example scenario: the home office summoned us from our respective positions in different foreign countries, in order to have us work together on a new initiative. We were summoned back home, in order to work together.
We are back, to work together.
Now, for a different example with a different meaning. We were formerly on the same team or in the same department, and we collaborated. Then we were assigned to different projects. Now we find ourselves working on the same project once again, and collaborating:
We are back working together.
Thus, back can mean "returned to a physical place" (the first example) or "once again" (the second example).
When back means once again, it modifies the verb-phrase.
When back means returned to a place, it modifies the subject. That's why in the first example, there is a comma after "back", to show that it is part of the predicate in the main clause, a predicate adjective.