Polis, Voluntary, Friendship

Many hold, and it is unclear if Aristotle is among them, that membership in the best Polis is voluntary.  But, at least one member of a family, a child, is involved involuntarily, and any association motivated by Economic necessity is also at least partly involuntary.  Instead, within Aristotle's system, the prototype of that concept of the best Polis is his one example of a voluntary community--Friendship, the political significance  he vaguely alludes to in the Nichomachean Ethics, but does not seem to explicitly formulate.