This comment is rather long but it attempts to fill in some of the gaps left by the Spark Notes web description.
In the Critique of Dialectical Reason Sartre explains that the practico-inert, as man-made physical substance, is the major force in maintaining social control when run by hegemonies seeking to limit human freedom. Sartre conceives a notional progression running from the lone individual, to the fusing group, to the pledged group, to the organisation, to the ossified institutions that surround us all, thus giving the reader powerful insights into the structure and dynamics of human societies. These notions are fundamental and it is not necessary to view this depiction solely from a Marxist perspective (you need no background in Marx.)
The Critique describes the unassociated individual much as first given in Being & Nothingness. The lone individual experiences herself as unaffiliated inside a society where she has a serial condition existing in a state of alterity – the opposite of reciprocity – whereby each member of a series is linked to every other through disassociation and impotence. ‘Series’ is a special Sartrean term where each member of a society is determined by an essential otherness, running between one person and another. We must realise that Sartre considers negative states – absences – as constitutive of real relations in the world. Although each member is a part of a collective, there is nothing in physical proximity that bonds us in engaged association. Social alienation is thus defined by Sartre as a real condition defined by negation.
For Sartre, previously unassociated individuals may still form into groups through the groupe en fusion which is a kind of transformation when individuals realise they have goals in common and want to see them realised as a human practice. Cooperation now seems advantageous. For Sartre, it is the individual that realises a potentiality in the groupe en fusion and, along with others of a similar disposition, freely chooses to associate.
Once the group has been formed, it may further its aims by pledging and becoming an organisation which, in turn, uses the man-made physical world (the practico inert) to further its aims.
Within illiberal regimes, the young are the victims of a certain kind of mystification because they arrive inside institutions where previous generations have already defined what they will become as ‘determinations of untranscendability’.
For Sartre, simply by being born, one enters a pre-established pledge whereby one must accept one’s situationality as a plain fact to be unconditionally adopted along with the values and projects of others as one’s own non-optional future.
Yet Sartre’s philosophy, since the appearance of Being and Nothingness, insisted that the only ontological entities with free choice are individuals. He maintains one can oppose but cannot remove another’s freedom.
However, a commandeered structure of the practico-inert (through the media) guarantees the possibility of our manipulation within a serial isolation where an illusion of general consent is maintained—one where no general reciprocity ever existed in the first place.
Further information on untangling Sartre’s complex work is available from John Wilson at firstname.lastname@example.org