What is a group of elk called?

There isn't a specific name for a group of elk, unlike some other animals which have collective nouns (e.g., a murder of crows, a pride of lions). While there's no officially recognized term, people sometimes use terms like a herd of elk, which is generally understood. This reflects the fact that elk, like deer, tend to live in groups. The term "herd" applies well enough, though it doesn't carry any special evocative meaning like some other collective nouns.