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Beside battle fund, gold boxes, rank-ups, rank-ups and purchases of your refferals, daily rewards and your own purchases of crystal you can also obtain them in various competitions like "Video of the Week" and other actions organized by devs.
Is this not the right type of question for this Help area or why is it still in "AWAITING MODERATION" state?
AFAIK: That teammate gets 10*(number of players in other team in time of capturing) points, you get none. But your team gets one score point which makes you get more crystal for your obtained battle points in the end. Helping your teammates to capture flags is usually better for you than to wrestle over the flag with them and lose it.

Flags are returned when
  • somebody from given team rides over it,
  • flag gets to unallowed area (like when tank carrying it drives off the map)
  • after some time it is just standing in the middle of battlefield (like 20 seconds or so since the time last enemy player dropped it).