Given a class C, we may append it with the literal "group" to obtain a class of sets whose elements are instances of C, and which are related in some way.
If you're not super familiar with object oriented ontology, using the class "car" as an example, a "car group" is a set of related cars. The 2018 Toyota Camry is a car. The set of all cars designed in 2018 is a car group.
Some more examples are:
- An operating system group. The set of UNIX-like operating systems is an operating system group.
- A food group. The grains are a food group.
The question is: what part of speech, linguistically, is played by the word 'group'? It is not a modifier, as it cannot be removed and have the sentence remain syntactically valid. However, it also can be appended to any class and will "act on" the class in some way.