Many people that want to organize groups online, typically attempt to do this through a site dedicated to a specific topic, an inflammatory article/video, etc. This is a fairly arduous and suboptimal process. A better approach is to build a site that enables people to accomplish things. Software or sites that do this typically include:

A database of some sort. A collection of structured or unstructured data that can be used to do things. This spans classic databases, file servers, wikis...

A user interface that allows community participation. Typically this allows the community to add to the database, monitor/vote on additions, reputation, etc.

An open connection to the database (an open API) that allows new applications to be created. From mash-ups to iphone apps.

The benefit of this passive approach is that it creates the potential to form an open source community around the topic. As such, it not only attracts people tightly aligned with the creator's goals, but also a much larger group of individuals and groups that are pursuing tangential and marginally aligned interests. Also, because the site is a tool, it benefits from a virtuous feedback loop that tends to accelerate growth: people that use the site often see immediate benefit from the interaction.

Software tools of this type can be used to create open source communities for both positive endeavors (like building resilient communities) to insurgency.

For example, say I wanted to run an insurgency against financial capitalism, and in particular an effort that specifically targets Goldman Sachs. The software tool approach offers a variety of entry points for this endeavor. One aggressive methodology that enables corporate targeting would involve:

Assembling a public database of Goldman Sachs employees and alums. This database could either be unstructured data in a wiki, or more forcefully, a structured database that includes name, work title, e-mail, home address, phone number, etc. The key, in either approach is to seed the database with a critical mass of data to make it useful from the moment you launch it.

Creating a method by which additions, creations, and improvements can be made to the database via the community. Allow users to add info, from pictures of the individuals (cell phone stalking) to salary data and job roles...

Opening access to the data to allow an ecosystem to develop. Community developers could use the data to create a mash-up tool that allows users to see all of the homes for Goldman Sachs employees on a Google map (or equivalent). Another application could place employees into a model of the corporate hierarchy.

In sum: superempowerment can breed its own group dynamic.