As I understand it, when reciting the Shema by yourself, you add the words "Kel Melech Ne'eman" beforehand; but when praying with a minyan, you don't.
How "with the minyan" do you have to be? What if you start your Shema a minute earlier than everyone? A minute later? What if you have to daven fast, so you're up to Shema while they're still in Psukei D'Zimra (opening Psalms)? Or you got there late (due to some mitzva-related circumstance beyond your control) and you're saying Shema while they're almost done? Does it all depend on whether you hear the Chazan say the concluding "Hashem Elokeichem Emes"?