Groups USUALLY (depends on cruiseline) of a minimum of 16 passengers in 8 cabins. The group will get a free bed/berth (of most category booked), amenities (depends on ship, sailing, desperation of cruiseline, etc.) of onboard credit and/or cocktail party, photos, etc. You can form your own (usually best through a travel agent (no charge to you) or buy into an existing group (in which case you have no control over amenities). Absent the value of the goodies, there is no benefit to forming a small group over booking a cabin(s) on sale.