One of many definitions of people is "a body of persons unified by a common character, culture or sentiment." (Webster's New Collegiate Dictionary) We can refer to the French people, the Spanish people, etc. So, to answer the question posed in your title, peoples can be used as a plural, as in "the peoples of Europe" or the "indigenous peoples of the Americas." Please note the difference between peoples (plural) in your title and people's (possessive) as written in the body of your post.