What is the formal word for the plural you?
It is "Sie" and "Ihnen" etc. beginning with a capital letter.
Folgen Sie mir bitte? - Would you come please?
Wie geht es Ihnen? - How are you?
"ihr" and "euch" etc. is addressing second person plural informally.
"du" and "dein" etc.is for the second person singular informally.
Usually these are not capitalized,
unless used in formal writting, such as letters and it is subject to discussion if e-mail and texting count as formal.
No, I made a mistake here. It can be capitalized in all written form when the other person is addressed personally. But not in impersonal writting such as advertisements.