Some, like Genethas, most likely joined willingly due to the promise of power and being spared.
However, I think that the majority endured brainwash by dreadlords.
Considering the average intelligence of medieval peasant... dont think that brainwashing was a big deal, maybe something like "go with us and you wont starve"
We should keep in mind that kingdoms at that time probably didnt really have anything like professional army.