Dota 2 has a lot of pushing strategies, I can't describe them all to a tee but I'll do my best here to summarize a couple of them:
Ganking:
Most effective in the early stages of the game when your goal is to follow up with a tower push. The goal is simple: eliminate the opposing team's lane presence and pressure the tower with everything you have (usually 2 heroes + a middle rotation). Works remarkably well when your side lanes just hit level 6.
Summoning Heroes:
Super effective in the early stages of the game, this strategy consists of bringing 1 or more heroes that have summon-able/neutral creeps at their disposal to push the lane with your creep wave. Easily executable in the short lane with a well timed stack/pull to start the push, this strategy almost guarantees a tower kill in the early game if the opposing team doesn't teleport to defend it.
Here's a quick list of heroes who are well known to be used in the fashion (but not limited to):
If you're trying to execute this strategy, study your enemies wisely and look for good counter pushers. Dark Seer, Earthsharker and Tinker will make it hard on you to push in the early stages.
Split Pushing
One of my personal favorite mid-late game pushing stategy. This strategy, in a nutshell, consists of having as much of your team pushing a single lane while one other hero gives great momentum to creep waves in another lane. Nature's Prophet and Tinker (with Boots of Travel[BoT]) are kings at this game. Your team is not to team fight during this time as 4v5 or 3v5 engagements will rarely be in your favor.
Note it's also possible for you to have 1 hero push top lane, 1 hero push middle lane and the other 3 stalling bot lane in order to create more pressure on the opposing side. Out of this 1-1-3 split, it's possible to pick off enemy heroes that are trying to rotate to defend.
I'll finish by saying that split pushing is a delicate operation but can yield great results when executed with patience and discipline.
Hope this helps,
Happy Dota!
EDIT: Ever since the First Blood update, the behavior below is no longer applicable. Now captains are no longer randomly assigned. Instead, there is a button to allow anyone to make themselves the captain, first-clicked-first-served.
== OLD ANSWER ==
It is random based on the number of groups. What that means is, by grouping up as 4 and taking on one straggler, your group/captain has a 50% chance of being the drafter, and the solo guy has 50%. If you were to group as 3 and take on two other solos, it would be 33/33/33, whereas if you caught a pair of partied players with your trio, it would be back to 50/50. A pair and three solos would make 25/25/25/25, and so on...
Long story short, unless you can live with this quirk in the CM system, I do suggest getting a fifth in your stack. Otherwise, give Random Draft a try. It can be just as strategically interesting as CM.
Best Answer
First, you can use this site to see some recent professional games using Riki: http://www.dotacinema.com/vods ... simply go to Heroes, scroll to Agility, and select Riki. He's not incredibly popular, but here's a game from just 5 days ago where Alliance used him effectively: http://www.dotacinema.com/vods/8990#game1
The hero has been played 13 times (6 wins) in Patch 6.81, which notably buffed his backstab damage against allied units, making him able to heavily deny enemies in lane. You can see a list of games here: http://www.datdota.com/hero.php?q=Riki, or search for 6.81 specifically here: http://www.datdota.com/match_finder.php
So, Riki's primary strengths:
The main things in mind for me here are that you want to pick Riki alongside strong teamfight initiation so that people feel forced to run away. He's also strong against mobility like QoP, Storm, and Puck due to his silence plus Lifestealer and Anti-Mage for the same reason. With the typical Diffusal Blade pickup, he does well against mana-reliant heroes as well (such as Wraith King, as seen in the linked game above).
Because Riki is an invis hero, he almost definitely needs to be last picked to prevent counterpicks of silences or detection abilities. Making supports spend money on dust and sentries ties in to the greedy laning phase you usually pick along with him. The longer that supports can be kept from getting up Force Staff the scarier Riki is as a hero. After that point, he loses a lot of effectiveness and probably will need help to carry the game.