Who plays in Em? (or in E flat which, contrary to an apparent popular belief is not the same thing at all and isn't what was asked).
Well if you don't you miss out a lot of good tunes.
And, as someone said, whichever key is fairly irrelevant, it's just scales, innit?
Take, for example, the extremely well-known Rochdale Coconut Dance in Em:

Em Am Bm C D repeat
with the B part going G Am D G Am D G D C D G

Not hard.
It's inexplicity fashionable among a few to boast that they know nothing of music theory.
But no-one would walk onstage and attempt to act without a command of the grammar of the language the play was written in. Would they?
I'm not a very good guitar player either.
I feel compelled to work out and write out chords first even when I'm fairly certain by ear (or from the key mentions, if it is) what they should be.
I think that's a better plan than just just joining in blindly, apparently oblivious to the key other people are playing in.
That's what I call thick, not raising head above parapet and inquiring where to start.