Just running some examples through in my head, I might possibly change a collective noun I usually use in the singular to the plural with 'a', but not the other way round. For me, 'group' is usually plural while 'team' is usually singular, and I might say any of the following:
The team of horses is pulling the carriage
A team of horses is pulling the carriage
A team of horses are pulling the carriage
A group of horses are standing in the field
The group of horses are standing in the field
I cannot imagine myself saying 'The team of horses are pulling the carriage' in any circumstances, but I might use a singular 'group of horses is standing in the field' with either 'a' or 'the', should the situation seem to warrant it.