Norm_Goldman ,I answered on your other post too, but your question here is a little different

Up to 4 people can share a "pay per ride" Metrocard. That's just for convenience so you don't have to have 2 or more passes. If you think you might travel separately at any point, it's better for each person to have a pass

"Unlimited Metrocards" are only to be used by one person. If you try to use it a second time, it will not work (for at least 18 minutes)

And when ordering your Senior Metrocard,you can also choose the Easy Pay option by giving a crdeit card number, refills when either %15.00 or $10.00 balance is left on the card.
Never have to worry about whther you have enough on your card. Easy.