What are otter's as a group called?
A group of sea otters is known as a "raft". A raft typically contains 10 to 100 animals, with male rafts being larger than female ones. Sea otters may wrap themselves in kelp to keep from drifting out to sea when resting and eating.
Believe it or not, they're called a "romp" ;)
If you are asking to what group of animals otters belong, they are members of the weasel family - Mustelidae of the Order Carnivora.