The early resistance was formed mainly by surviving Starfleet personnel who were able to set up isolated cells that communicated through the use of the Betazoid's natural telepahy. Many members of these early cells were uncovered and killed but enough survived to continue to recruit and maintain an active resistance.
During the liberation of Betazed the Federation Alliance made the resistance aware of their plans to finally drive the Dominion from the planet after over a year of occupation. While the fleet engaged the forces defending the planet in space the resistance assaulted the Dominion's military headquarters in Medara. Using ultritium rockets and isomagnetic disintegrators they destroyed the facility. This proved to be a crucial moment as it allowed the Federation Alliance fleet to gain the upperhand and defend the defending Dominion, Cardassian and Breen vessels. (The Dominion War Sourcebook: The Fires of Armageddon)