ok this is where knowing the modes comes in really handy

The chord Em fits in lots of keys, Em Am Bm

so u could play Em Am and Bm pentantonics and it would work

think of it like this if your given a minor chord to jam over say Cminor you can play that position pentatonic, perfect 4th pentatonic (up 5 frets), and perfect 5th pentatonic (up 7 frets)

all of these positions will be in key, the second two will sound a little different but theres nothing wrong with that

in a jazzy/ bluesy context you can play the pent box up a minor 3rd (3 frets) thats a cool sound