Yes, you can add up to 3 others to group, making for a total of 4 people in a group.

The results will come out within a week from now. Prizes will be sent shortly after.
Read this for more:
 

To get that symbol, you need to use the boosted supply shown on different drones, such as Double Damage for Booster, and Double Armor for Defender. But the "boosted" supply time lasts 3 seconds with Booster and 30 seconds with Defender.
 
Then there's Crisis, where just using this supply gives a slightly stronger supply than normal (however weaker effect than Defender or Booster effect, for example). Crisis can also give you boosted speed, if you use the speed supply.
 
Keep in mind, that per boosted supply, comes more supplies of that type being used. (3 extra used supplies for Booster, 3 extra used supplies for Defender and 5 extra used supplies for Crisis).

Tanki Online doesn't reveal that information to players, because it's a piece of "security" for some reason. I don't know why though.