Is an indefinite article necessary in this context: "A group of [...] (a) group which [...]"?

Exact context is:
"… a number of people had been demanding […], (a) group which became more vocal as […]"
Alternatively:
"… a group of people had been demanding […], (a) group which became more vocal as […]"

Personally I believe omitting the indefinite article makes the sentence flow better, but a friend has suggested it is incorrect. Are there any rules on the inclusion of articles in this context?

Thank you.