If part of party doesn't show for flight?

[Lewisc]

While it is true that you can get your miles redeposited to your account for a cancelled FF ticket, there is usually a fee to do so and it's usually the same fee you would pay to change a ticket. On United, it's $150.00, unless you are a 1K or GS; then the fee is waved.

It would be best to buy a refundable ticket for iffy travelers.

Actually it's best if you let iffy travelers buy their own tickets. Otherwise the suggestion of using FF miles is good, you just need to use an airline with low (or no) redeposit fees.