Never share food by passing from chopsticks to chopsticks because this resembles a custom at Japanese funerals when cremated bones are ceremoniously transfered to the urn. This is probably the biggest taboo at the Japanese dinner table. You can transfer food using your chopsticks to someone else's plate but get them to pass the plate to you if it is a distance. Ideally you would ask the server for another pair of chopsticks that are placed in the center of the table and used whenever someone needs to transfer food for another person.