Dota 2 players are going to be able to partake in some ranked multiplayer matchmaking soon (in the next major update), after Valve announced the new feature on the Dota 2 blog.
The blog also explains how they calculate each player's Matchmaking Rating (MMR), and how that information is used to create balanced matches:
"After each match, we update your MMR based on what happened in that match. In general, when you win, your MMR will go up, and when you lose, your MMR will go down. Win/loss is the primary criteria used to update MMR, but individual performance also plays a role, especially when our uncertainty about your MMR is high."
These calculations (along with others) will be done across four different modes; normal solo and normal party, ranked solo and ranked party.
Ranked matchmaking will be available once a player has completed approximately 150 games, and all players in a party must have unlocked it or the whole group won't be eligible. Again, for more information on how the new feature will work, head to the blog.