"You all" is sort of the way people teach English speakers the difference between singular and plural terms, but I don't think duolingo actually recognizes "you all" counts for the one plural word for "you." It just assumes you know that it's the plural you. I don't know how you do it, still too new, but you could report it and request them to add "you all" as valid. But, technically, it isn't "you all." It's just referring to a whole group of people (instead of one person) when it says "you."
Both came from german "sein" On link bellow is info about how it is conjugated: http://german.about.com/library/verbs/blverb01.htm
Most often you can determine whether it is 3rd pers. sing. fem. or 3rd pers. plural by taking a look at the inflected verb:
Sie ist langsam. vs. Sie sind langsam.
The polite form 2nd pers. looks like 3rd pers. plural except for the capitalisation: comp. sie (they) / Sie (you):
Ich denke, Sie sind langsam. (sing. polite) vs. Ich denke, sie sind langsam. (plural)
"du bist" means "you are" and "er sein" doesn't really make sense; it sounds like "he be" so I'm assuming you meant "ihr seid" which means "you are"
The difference is that "du" is singular and addresses one listener and "ihr" is plural and addresses multiple listeners (i.e. "you all are")