The Resistance movement was formed after the Arrival. They began launching raids and assaults against the Redhats, in order to end the occupation in Los Angeles. There was also a resistance group in the occupied Seattle, before they were quelled by the occupation. In the episode called, "Tamam Shud", it is revealed that there are more resistance groups outside of the colony.
For a time, about a year after The Arrival, a figure called Geronimo appeared to be coordinating The Resistance, he made regular and very popular radio broadcasts, a lot of very professional looking posters also appeared. To add to the mystery, the times and radio frequencies of the radio broadcasts were hidden in code on the posters. Even within The Resistance, Geronimo was presented as an important figure by one-time Cell Leader Alexander Quayle. It turned out that the man making the broadcasts was an actor, Luis Ortega, who'd been hired by an advertising executive living in the Green Zone. The ad executive was behind both the scripts for the radio broadcasts and the posters - but did not appear to have any actual connection with any resistance groups