When 'alternativereward' is set to true, the experience and money you will get for killing a dragon will be splittet between all the remaining players in that end world. (Only those that did damage to the dragon!)
If a player (player A) does 80% of the damage and another player (player B) does 20% and kills the dragon:
Player A gets 80% of XP and money, Player B gets 20% XP and money ...
A player who dies before the dragon was killed has bad luck and will get nothing!
The dragons drops (egg, head) will drop as usual...
(if you want to make sure the killer gets a bonus, you can use the commands for that!)