Gei is a prefix particle that means 'group of'. It functions similarly to yei and tei. It's most commonly used in numbers. For example geitu is a twosome or a couple; geivi is a threesome; geisa is a foursome; geixotu is a dozen; etc etc.
Gei can also be used in other ways!
Geikomodenusa - A litter of kittens
Geihakaxasa - A circle of friends
Geibiho[muka]sa - Cattle
Geixudosa - A school of fish
A plural is always required when gei- is being used with a non-number word, generally a noun. -Sa is most common, but if we want to draw attention to the individuals in the group being different we can use -sai.
Geihemodosai - A group of unique individuals (people)
Geixudosai - A school of fishes