Complex sentence: In Chiprovtsi, they are thought to have arrived as a single group of around 50--60 people with their families recruited by means of special contracts and receiving some privileges compared to the natives.
Split into simpler sentences: In Chiprovtsi, they are thought to have arrived as a single group of around 50--60 people with their families. They were probably recruited by means of special contracts and very likely received some privileges compared to the natives.