working group vs social unit

working group and social unit both are nouns.

Word	Noun	Adjective	Verb	Adverb
working group	Yes	No	No	No
social unit	Yes	No	No	No

As nouns, social unit is a hypernym of working group; that is, social unit is a word with a broader meaning than working group:

working group: a group of people working together temporarily until some goal is achieved
social unit: an organization regarded as part of a larger social group

Other hypernyms of working group include unit.

working group (noun)	social unit (noun)
a group of people working together temporarily until some goal is achieved	an organization regarded as part of a larger social group