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In the following sentence:
En entrant, je suis tombé sur un groupe de personnes bruyantes.
I thought since the personnes is plural, the article must be either les or des. So in any case the article is concatenated with de, to become des (un group des personnes).
However, the sentence uses de, not des.
I have learned that some of the expressions that are related to the quantity always use de, irrespective of any articles following de. These expressions include beaucoup de, un peu de, or autant de.
Is the un groupe de in such a group? Otherwise, why does the sentence above not take the article to make it un groupe des?