This system is being added in patch 1.0.5 (it is referred to as Monster Power). Details are available on the official Diablo III blog .
You first need to select "Enable Monster Power Selection" on the game play tab of Options. You will then be able to select your desired monster power level from 0 (off) to 10 in the Quest Selection interface.
Here is a quote from the blog regarding the "/players 8" asked above.
Similar to the "Players X" command in Diablo II, Monster Power gives Diablo III players the option to increase the health and damage of monsters according to different "power levels" -- and in return, they'll receive scaling bonuses to their adventure stats (including experience, Magic Find, and Gold Find) as well as more drops.
Here are some of the charts as of 1.0.5:
UPDATE: An updated version of the Inferno chart for 1.0.7: http://us.battle.net/d3/en/forum/topic/8920082006