A ghost must be traveling a similar bearing, speed, altitude, and be in close proximity - for a period of time. If you take off with a wingman, he will look like a ghost. If you meet in the air and join up, it will take a long time before the wingman is considered a ghost.
BTW, another option for wingman is to tap the aircraft and select the "Track the Traffic" option. This will highlight the icon a little differently, remove that guy from the alerting system, and he won't ever be considered a "ghost".
-Walter