Post-left anarchists do not work to create mass structures. This does not mean not working exclusively at an affinity group level, but that whatever organization is larger than that is very temporary. Perhaps created for one action, and disolving hours later.
To a leftist, this is not "organization," by which they mean to indicate the establishment of an institution which becomes more than its members. We are not concerned with creating broad-based movements, but we do participate in larger movements, at our best as a destabilizing element for those who are attempting to use said movement for their own political goals.
The problem with "mass structure," as I am interpretting it in this context, is that it takes on a life of its' own, and individual needs and desires are subsumed to the good of the mass. For myself, this is the point at which organization becomes a problem: acting not in our own interests but to perpetuate something more (I think the classic @'s would've said "the idea"). Ideas are important, but they are not more important than my life, or yours.
The model outlined as autonomous base nucleii (http://theanarchistlibrary.org/HTML/o.v.__Autonomous_Base_Nucleus.html
) is defintiely one of the forms of organization post-leftists might use, if it was appropriate to the situation. For myself, part of identifying with the post-leftist strain of thinking is a willingness to not be wed to any particular way of doing things. What is tactical? What will make sense?
There are even times when I choose to critically engage in struggles for the sake of delivering a different critique or analysis than that of the "organizers."
Does this at least help to answer the question? I feel as if I am leaving something out that I tacitly understand but am unable to put my finger on, so please ask further questions.