[yggda]

The way I handle this in the Bay Area is keep an extra autoload Clipper card for any guests that are coming to visit. hand it off to them when they get here. It's pretty easy to let them know how much to settle up for when they depart.

I'm not sure how most visitors to the bay get in but two desks at SFO and OAK across from baggage claim seem like a good start to implement the same kind of solution.

[ryandrake]

Or, for an ethical gray-area solution if your guest has only a one way trip (like to the airport): give them a Clipper card with some minimal ($2) balance. They should be able to go to their destination regardless of the fare. The card’s balance will go negative, but they can just throw it away.

[nitrogen]

The BART gates won't let you out if you don't have enough money on a ticket, so you'd have to jump the gate or add funds at a kiosk (think the same applies for cards, too).