"Byl jste tady nový."

[ion1122]

Here "byl" is singular, so "jste" cannot be you plural; it must be you singular polite form!

[tatka_]

"byl" is singular and "jste" is formal singular (or plural). Please read Tips and notes: https://www.duolingo.com/skill/cs/To-be-plural

The sentence for plural is "Byli jste tady noví."

[JanLyko]

Exactly! Accordingly "Byla jsi / byly jste / byla jste".