The Ruulian Rebels were definitely in operation as of 3 ABY, when a new outbreak of rebellion swept across Ruul independent of the Alliance. Dellis Tantor was sent by Grand General Malcor Brashin to squelch the revolt, but Tantor's assignment was simply a ruse to arrange his capture.
Whether any actual action was taken or not, the Ruulian Rebels were still fighting the Empire when Brenn Tantor, Dellis's older brother, arrived with a small Rebel strike force. Tantor's goal was to free Beri Tulon, who was being marched to Tarkin Detention Facility, but Tantor also managed to free a Ruulian Rebel leader who was being escorted along the same route. Several other Ruulian Rebels were in the area, possibly planning a rescue mission of their own; the leader was returned to them.
Tantor followed up the rescue with an information raid on the Sounder Flats computer complex. Several Ruulian Rebels were in the area, including some with access to stolen AT-ST and AT-PT walkers. Tantor was able to recruit these guerrillas and use them in the raid. Presumably, the Rebellion offered them aid after its success at the Battle of Endor, which occurred shortly after Tantor's mission.