Don’t have enough airline miles for that free flight? United Airlines is now letting people pool and share their frequent-flyer points with family and friends, a feature currently offered by some smaller carriers. United said Thursday that a “pool leader” can pick up to four other people to set up a joint account in its MileagePlus program. The group leader must be over 18, but there is no minimum age for others, so parents can sign up kids. Everyone in the pool must have their own United frequent-flyer acc ...