(through Online Etymology)

The pronoun of the second person in English grammar used to break down like this:

Nominative singular: THOU
Nominative plural: YE
Objective singular: THEE
Objective plural: YOU

In the Middle Ages, people began to use plural forms in all cases, at first as a sign of respect to superiors, then as a courtesy to equals. By the 1600s, the singular forms had come to represent familiarity and lack of status, and fell from use except in the case of a few dialects, notably in the industrial north of England. People in Lancashire north of the Rossendale Forest and Yorkshire used to be well known for their use of the singular second person pronouns tha (nom.) and thee (acc.). For religious reasons, the Quakers also retained the familiar forms, though generally in such a way that thee was used in all cases, along with the third person of the verb (thee has where grammar would dictate thou hast), and they brought it to America, where it was current in entire neighborhoods of Philadelphia till the 1890s and in some farms in the hinterland for perhaps another generation after that.

Yes, surprised too, that YE / YOU once was a plural form.