Does the IGF's open membership mean that it can't legitimately make decisions? It does mean that it can't make them democratically in the representative sense. But neither can anyone else, because there is no transnational polity that represents all four of the stakeholder groups; governments, the private sector, civil society and intergovernmental organisations. The United Nations certainly doesn't. And yet public policy decisions of transnational scope affecting all stakeholders do have to be made.
So what's the answer? Perhaps to take a broader conception of what democracy is, more concerned with public accountability and inclusiveness than proportional representation, and which extends to all four stakeholder groups, both individually and together. David Held is one scholar who writes on this so-called cosmopolitan conception of democracy, and he says:
Democracy can only be fully sustained in and through the agencies and organizations which form an element of and yet cut across the territorial boundaries of the nation-state. The possibility of democracy today must, in short, be linked to an expanding framework of democratic states and agencies.
So on that basis, the fact that the IGF's membership is completely open makes it easier for it to develop a legitimate role for itself, because everyone who is affected can be involved, in an accountable and transparent forum.
Now of course, that still means that the institutions of representative democracy such as voting won't work. But that is precisely when consensus as a means of decision making becomes most useful. When reasoned consensus is required to make a decision, it is impossible to stack a vote. One reasoned argument can be sufficient to change the decision-making process, so vote stacking becomes superfluous.