What does bist du bei mir mean in English?
"Are you with me?"
That would be a translation of the question "Bist du bei
mir?".
The start of the sentence as in the Bach/Stölzel aria "Bist du
bei mir, geh' ich mit Freuden" would be better translated as "If
you are with me (or: if you're near to me), I go light-hearted (or:
with gladness)".