I think this is an important challenge to theories of democratic authority. I think it reflects an implicit concern of anarchists, namely, that there is nothing in current political institutions that justify the “we” many use to legitimize State rule.
If the democratic process is a means by which some collection of persons may rightfully govern itself, what constitutes an appropriate collection of persons for employing the democratic process? Is any collection of persons entitled to the democratic process? In short, if democracy means government by the people, what constitutes "a people"? There may be no problem in the whole domain of democratic theory and practice more intractable than the one posed by this innocent-seeming question. To grasp it, imagine an aggregate of persons. Adapting Jonathan Swift to our purposes, let us call them the Eggfolk. While many Eggfolk contend that the Eggfolk constitute a single "people," some insist that they are really divided into two distinct peoples, the Big Eggfolk and the Little Eggfolk, with such different ways and beliefs that they should govern themselves separately, each entitled to its own fully democratic system. How are we to decide'? As we shall discover in chapter 13, democratic theory supplies little by way of an answer. In fact, while historical answers exist, there may be no satisfactory theoretical solution to this problem. Robert Dahl, Democracy and Its Critics 116-17.
Note that I am still in the elementary stages of my own journey through democratic theory. Forgive me if there are simple answers to this problem, or if I change my tune throughout the coming months. But in the spirit of philosophy, I comment on that which I am currently dealing. I think there is value in actively engaging the arguments you are currently dealing with, even though you have not digested the entire (or even a sufficient amount of) domain. As long as one realizes his own ignorance vis-à-vis other philosophers and lines of argument, taking part in the conversation is highly beneficial.