Ad hoc literally means "for this." In context, it generally means something that's put together for the sake of something specific, rather than permanently. Ad hoc committees tend to be temporary, or made up as the need arises. In this case, many are coming for the World Bank protests, and many community activists felt a need to provide a space for these protests, to let them know that something indeed was going on. MGJ will be working on these protests, but MGJ has decided not to undertake a mass mobilization or to make the World Bank protests the centerpiece of its strategy. As a result, there is a bit of a vacuum. There's a need "for this." So, a collective group of people formed the ad hoc committeed known as the Farragut Squares.