Social groups? Sure. Armed rebel groups in the middle of an insurgency? Disloyalty is .. less tolerated.
It was the middle of an insurgency, but these groups were fighting to suppress it, not support it.
Orwell was in a roughly similar role to one of the idealistic Westerners going to fight in Ukraine today, except the anti-Fascist side was much more politically fragmented, less well armed, and ultimately lost, leaving Spain as officially Fascist until 1977.