Memoir versus Narrative Nonfiction

A Guide to Literary Agents "editors blog" definition describing the difference between memoir and narrative nonfiction is: Memoir is when someone writes about their own life. Narrative nonfiction is when someone writes about the lives of others.

Seems a little simplified...For instance: What is a story about you and others around you experiencing a common coming-of-age experience? Is it a memoir? Or a narrative nonfiction? Or both?

It seems to me it would be hard to write a memoir without including others who shared that same time and space with you.

Or is a narrative nonfiction a book researched by an author and is based on true events but the author was not involved in the events?

Please give this confused person some input...Thanks