Good point! So "ye" would count as a formal middle English pluralisation of you/thou, and is still used in a few places today. A little searching on Wiki turned up some additional interesting informal cases: https://en.wikipedia.org/wiki/You#Informal_plural_forms
Du = (Singular) "You" subject. Deg = (Singular) "You" object. Dere = (Plural) "You" subject/object (works for both). Same meaning as "y'all".
Examples: You like us. (You plural & subject) ---> Dere liker oss. They love you. (You plural & object) ---> De elsker dere. You like him. (You singular & subject). ---> Du liker ham. I love you. (You singular & object) ---> Jeg elsker deg.
Sorry, the formatting was bad.
Of course, I'm assuming that you know what are objects and subjects in English. Here's a detailed explanation
In English you can talk about both one or more people using You, and even more conveniently, it can be used as both subjects and objects. In Norwegian you have to differentiate between them.
Du liker oss: You like us. (Here, you is singular and is also a subject)
Jeg elsker deg: I love you. (You is singular & an object)
Dere liker ham: You like him. (You is plural & a subject)
De elsker dere: They love you. (You is plural & an object)
So, basically, dere can be used as both a subject and an object (it doesn't change) when you're talking to more than one person. But du is only used as a subject, the object form is deg, and it's only used when you are talking to ONE person.