How much you pay in tax is entirely based on what tax bracket you fall under. You are only taxed on your enrolled friends travel, not yours or your family.

It is payroll deducted in the form of income taxes withheld.

Lets say you earned $2,000 for the paycheck, your secret girlfirend i.e. *enrolled friend* flew to Omaha for a overnight rendezvous with you a few weeks back - Yonited calculates a fare value lets say $200 - which the IRS says Hey! That's a taxable benefit! - So that "value" is represented as "imputed income" on your paycheck - NOW your paycheck says you earned $2,200 and your taxed on that $2,200 according to your with holdings, exemptions, etc....get it