Cell are a resistance group which appears as an enemy faction in Crackdown 2.
After Catalina Thorne crippled the Agency by destroying their superhuman and cloning programs, she convinced the people of Pacific City that she was the solution. Citizens started to follow her and they joined her group called Cell. This faction mostly consists of citizens who hate the Agency's policies and people who want the Agency to find a cure for the Freak virus.
Cell members engage in combat with Agents, Freaks and peacekeepers on sight, and protect the vulnerable residents of the city in special 'Tactical Locations' they created. They seem to be unaware that their leader was indirectly responsible for the creation of the Freaks so she could get followers and take over the city.
After the events of Crackdown 2, it is likely that Cell either dismantled or the surviving members reformed into a smaller group.