Mr. PBQ has access to the Emerald aisle. I do not.
He is arriving at a different time. If he adds me as an additional driver, can I pick up the rental car?
Nope, primary renter has to pick up the car. Anyone can get access to the Emerald aisle with a free Emerald Club account. The Executive aisle is what requires status.
Sometimes, there's little difference between the two, but typical differences are as follows:
Emerald Aisle: Mostly intermediates and standards, occasionally some full size and intermediate SUVs.
Executive Aisle: Full size and above, with premiums, SUVs, minivans, and occasionally a few luxury or premium crossover choices.