Group Attack
When launching an attack, there will be an option on your Fleet page called "Formation". If Clicked on a window opens giving the attack launcher the option to send invites to themselves, their buddies, and their alliance, together with a custom message.
On opening the message the receiver is shown the message, and given the option to "Join Group Attack". Anyone that accepts invitation by clicking on "join Group Attack" has the standards "attack" message box open allowing them to select the fleet they wish to use.
(Note: it is not possible to invite the owner of the target planet to participate in a Group Attack against themselves.)
If a invited party is further away than you, this will delay the time of the attack until the late arriving party would arrive. It is believed that anyone who would cause a delay of more than 30% of the original attack time will not be able to join.
Additionally, if you are not within attack rank range (ie too high of a rank or too low of a rank vs the person being attacked) you will not be accepted into the formation even after accepting the 'join attack' request. Check the colony being attacked first to see if you would 'normally' be able to attack before trying to join a formation... if you can not attack as an individual because of the difference in rank then you can not participate in the group attack either.
If the formation leader recalls, the invitees will still attack. The attack simply changes from a group attack to a normal one. This guarantees that return times of the invitees can not be tampered with by the formation leader. Also, since the formation leader isn't 'locked in', he can recall to defend his base with out cancelling the other players' attacks.
A maximum of 6 fleets may participate in a Group Attack. This means a player initiates a group attack, and 6 other fleets may participate, making a total of 7. If ANY of the attacking fleets' home planets, or the planet under attack, are scanned by an Oracle, the Oracle owner can see all ships participating in the group attack.
Its defensive counterpart is Group Defend.
Note: Ships present at a planet during a Group Defend mission's orbital period will combat against any incoming attacks, INCLUDING those launched by the Group Defend fleet's owner and/or their allies and/or their buddies. This, in fact, is the only way to fire upon your own ships.
Split
Spoils are divided equally among all ships' cargo holds. If you both have 10 Hercs, for example, you will get an equal share, not counting other ships including attack ships. If you have 6 and the other guy 10 then the other guy gets more. Keep in mind, if you lose a Herc leaving you 9 and the other guy still has 10, they will take more. Also remember attacking ships have cargo capacity so they maybe get a bit more from that. Also if you use up more than one slot on the Group Attack list, it seems that you will get a extra shares of the profits.
Players' differing battle techs (weapons/armor/shields) effect on the battle outcome is currently an unanswered question.
