A student once commented to me that although he could easily understand and use singular and plural very well, the use of partitives (words used to refer to a group of something) was a frequent consternation. Hosts of students of English have marveled as the battery of vocabulary memorization necessary to properly learn how to refer to things in groups. So, I dedicate this G-Spot to partitives.
To form a partitive we use the word 'of' plus the plural form of the object to which we are referring. Certain words are commonly used to refer to a variety of things or groups of people and while this is by no means a comprehensive list, it does offer some options that can be used.
Assortment
Batch
Battery
Cluster
Collection
Crop
Group
Host
Selection
Set
Variety
For amorphous Groups of people you can use the following:
Army
Band
Crowd
Gathering
Horde
Knot
Party
Swarm
Throng
If the group of people or things have a certain shape then the following can be used. In general, these words make sense because the word used represents the shape or position of the objects:
Circle or Ring - refers to a group of things or people that form a circle or similar shape
Jumble, Scatter, Scattering - refer more to the lack of shape in the group
Sprinkling - would look like a bunch of nuts dropped on a table with no certain pattern.
Pile, Heap, Mound, Mountain - refer to how things are gathered in a representative shape.
Column, Row, String, Stacking, Line - suggests the way in which things appear to be ordered.
If groups of people or things have some kind of movement or if they occur with a certain frequency then the following can be used:
Hail, Barrage, Shower - suggest things coming from above or possibly being thrown.
Flood, Stream, Tide - normally refer to water but with groups they suggest a fluid movement of the objects or people.
Rash, Spate - something that happens suddenly and in large numbers.
Series - something that happens with some regularity
Volley - might suggest a short but controlled 'bombardment' of something (a volley of gunfire)
Perhaps the most diabolical group of partitives comes when discussing animals. Most animals have an appropriate partitive that is 'correct' however most can be referred to as "a group of _____" if you lack the vocabulary and in truth most native speakers would only be able to name about half of the following so don't stress about it too much:
an army of ants
a swarm of bees
a flock/flight of birds
a herd of cattle, deer, elephants
a litter of cubs, puppies, kittens
a school of dolphins, (fish)
a shoal of fish (not common)
a pack of hounds, wolves
a flock of sheep
a troop of monkeys
a gaggle of geese
a swarm/colony of insects
a pride of lions
a gaggle of geese
a murder of crows
a parliament of owls
(for more on birds, which are perhaps the most difficult, look here)
Some other notables for objects besides animals that you might see often:
a company of actors
a troup of actors
a wad/roll of banknotes
a fleet of ships
an army of volunteers
a gang/band/pack of theives
a flight of steps/stairs
a bunch/cluster of grapes/bananas
a bouquet/bunch of flowers
a squadron of fighter planes
a team/panel of experts
(source:Collins Cobuild "English Usage" Dictionary)
It should be noted that certain words do have 'connotations' or 'nuances' that would make them impractical in some situations and poingant in others, for example:
An army of nuns entered the church in supplicant prayer. - would seem to suggest to the reader that they are somehow organized and might even imply that they are somehow angry or militant; a situation that might seem comical to the reader in the context of 'supplicant prayer.'
on the other hand:
The professor had a mountain of papers to grade and a hoard of students waiting outside his door to hear about their final grades. - Uses a couple of descriptive partitives that not only suggest to the reader the scene but perhaps the feeling and even a certain urgency.
This is stylistic usage that usually comes from years of reading and/or writing literature.