Group nouns take plural verbs when you are conscious of each member of them such as "His family are all good at playing succer,"
This means any members are good at playing soccer.
But when you see them as a whole it takes a singular verb. "His family is good at tennis.
And Tunaafi says "a + group noun".means you are seeing the group noun as a whole, so that the group noun only takes a singular verb.
The construction of "a + noun" shows that you take it as a single thing.