You can use the Hit Man system to place a contract on a fellow player by finding the icon on the map. There are a number of places to go to access the system, you are required to place a minimum amount of cash which is $25,000 up to as much as you wish for the reward to place a contract on a player with a small amount of text as for why you are placing the contract. As for player wishing to take part in the contract that has been placing on the board. They are required to pay a minimum amount of $25,000 to take out the contract you will then be given a target to find and assassinate, once you have found the target and assassinated them you are to return to the Hitman system and claim the reward. However be aware that other Hitmen can take the contract as well and go after the same target the more Hitmen that go after the same target the more the reward goes up.
This job is illegal and Cops will come after you.