I think you can have multiple SmarTrips on your phone, but it might be a bit of a hassle to switch between them and tap/retap.
You can't tap more than one passenger in for a trip at a time, though -- because the fares vary depending on the specific trip (time/distance), it needs to track each entry separately, same as with the cards.
It's pretty easy for your guests to just download the app and get a card on their phone, but if you want to pay for their fares, you might have to buy additional cards for your own phone's app instead.