I don't think I have ever heard anyone just say "you" as a standalone when referring to more than one person in spoken conversation.
I live somewhere in Lancaster County PA and here most people say Yous, Yous guys, Yous both, You all, Y'all, All y'all, All yous guys and sometimes yinz but I think that was just when I was living north west of Harrisburg.
Hearing "You" as a standalone in a sentence like "I need you" however gramatically correct, sounds very strangr and almost foreign. Like If I heard someone around hear say that I would assume they were just talking to a specific person.
I really wish Duo would specify when they mean you plural and you singular because in spoken english wherever I have lived and I have lived in many different states from Florida to Texas to Arizona amd a few other places. I have always heard people specify some way or another gramatically between singular and ploral forms of "You"
I don't know if this is just the US or how other english speaking countries do it though.
"Euch" is either the accusative case or the dative case form of the plural "ihr", while "ihre" a possessive pronoun where the owners of the owned thing are being referred to in plural 2nd person, or there is just owner who is female and is being referred to in 3rd person.
"Euch" can be either accusative or dative case. Example of "euch" in accusative case:
Ich brauche euch = I need you (plural)
Example of euch in dative case:
Ich helfe euch = I am helping you (plural) / I help you (plural)
"Ihr" can be either nominative case or dative case. Example of "ihr" in nominative case:
Ihr seid toll = you (plural) are great
Example of "ihr" in dative case:
Ich helfe ihr = I am helping her / I help her
"Du" can only be in nominative case. Here's an example:
Du bist toll = you (singular) are great
"Sie" is the most versatile personal pronoun. The cases it can be are nominative and accusative, and the aspects it can have are third person female, third person plural, and 2nd person formal (which can be both singular and plural, like the English "you").
Here's an example of third person female in nominative case:
Sie ist toll = She is great
Third person female in accusative case:
Ich brauche sie = I need her
Third person plural in nominative case:
Sie sind toll = They are great
Third person plural in accusative case:
Ich brauche sie = I need them
Yes, the previous example is the same as another example. You'll need context to differentiate third person plural in accusative case from third person female in accusative case. Here's an example of 2nd person formal in nominative case:
Sie sind toll = you (singular OR plural) are great
2nd person formal in accusative case:
Ich brauche Sie = I need you (singular OR plural)
As you can see, third person plural and 2nd person formal are the same. The only difference lies in its written form, which is that the initial letter of the formal 2nd person pronoun is always capitalized. When it's in spoken form, you'll need context to differentiate them, but since it's spoken, you most likely have that context anyway. In written form, you need context only when "sie" or "Sie" begins a sentence, because the initial letter will be capitalized no matter which aspect it carries.
"Euch" is the accusative form of "ihr". When the pronoun is the subject of the sentence, use the nominative case "ihr", e.g. Ihr habt Bücher "You (pl.) have books." When the pronoun is the object of the sentence, use the accusative "euch", e.g. Sie mag euch "She likes you (pl.)."
In terms of "du", the nominative case is "du" and the accusative is "dich". The uses of these are the same as "ihr" and "euch", but mean "you" in the singular, informal sense.
Hope this helps!
Y'all hasn't been lost; it is a relatively modern invention. Originally English had 'thou' as the 2nd person singular pronoun, and 'ye' as the 2nd person plural pronoun. 'Thou' was informal, familiar, just like 'du' in German (you can see the similarity). It even has similar conjugations: Thou hast – Du hast; Thou goest – Du gehst. Not surprising since they have the same origin.
Later the plural 'ye' started to be used as a formal, respectful way of addressing people, also in the singular, and it changed to 'you'. Now 'thou' has almost disappeared except in some dialects, and therefore the distinction between singular and plural 'you' has also disappeared. In some regions they try to compensate this with inventions like y'all, you all, etc. I wonder why not the original 'ye'? :)
du = you singular informal in nominative (i.e. subject position)
ihr can have different meanings depending on whether it is used as personal pronoun or possesive pronoun:
ihr = you plural informal in nominative (i.e. subject position)
ihr = her (possessive* or dative)
ihre = their (possessive)*
Ihr (with capital) = your (possessive, singular or plural) formal*
(*) these are inflected according to the noun they are applied to.