Can we use a/an for plural pronouns?
Solution 1:
The tricky thing to understand here is that, while both "they are good people" and "they are a good people" are perfectly valid, they mean different things.
"They are good people' means that the group of people comprising "they" are, individually, good people.
But when you use the construction "a people" you are assumed to be referring not to the individuals but to the community. "They are a good people" would thus imply, if you are discussing citizens of the city of Mamatuti, that it was in the character of all Mamatitians to be "good".