What kind of a team setup is required to kill Roshan before the match begins?

With good control, any 5 heroes can kill Roshan before the match begins. It takes a good amount of teamwork and some money spent on health potions, however, and is easily punished if the opposing team realizes it. Because of this, the strategy is rarely seen.

There are certain heroes who are more effective at killing Roshan because of how their abilities work at level 1, and the increased speed or less punishment taken makes level 1 Roshan kills more common when some or multiple of the following are on a team:

• Ursa (Fury Swipes stacks indefinitely)
• Auras (Skeleton King, Luna, Drow, Beastmaster, Shadowfiend)
• Lich (Frost Armor slows Roshan attack)
• Venomancer (Wards can deal damage)
• Vengeful Spirit (Howl reduces armor by 2)
• Viper (Corrosive skin damages and slows attack speed)
• Any summoner plus Enigma. (Beastmaster, Nature's Prophet, Lycan, etc.). You can convert that summon into Eidolons and use further summons to damage, depending on hero.
• Lone Druid
• Wisp (Double effectiveness of Healing Salves with Tether)
• Timbersaw (Reactive Armor)

The reward is significant. Heroes will be halfway to level 3 if all split the experience, or if coordinated well one hero can be the only one within experience range and jump all the way to level 5. Each hero receives 200 gold and the killer gets a gold bounty of between 105-600 bonus gold. And, of course, the Aegis of the Immortal drops.

Items that help:

• Ring of Basilius for the +2 armor aura is an obvious one.
• Healing Salves. You can take turns tanking Roshan and healing quickly thereafter.
• Smoke of Deceit. If you're coming from the Radiant side, you're likely to be spotted without this. It also wears off when the enemy is nearby so you can know if you've been spotted on the way in. It also increases movement speed to get you there faster.
• Stats and regen. You are going to lane after this, and the bonus health and damage helps here too.

It's worth noting that the most popular combo recently is simply Ursa plus Skeleton King. Two coordinated players can kill Roshan by themselves with these heroes. Here's a video walkthrough I made to doing this:

Ursa is 100% the core hero you need to be able to do this during the pre-game timer. The easiest set up thus far in the beta (taking into consideration which heroes have been implemented) is to pick Ursa + Skeleton King. For your first skill, you need to pick Fury Swipes and Vampiric Aura.

Make sure to buy at least one Salve each (healing potion) and head to Roshan. Have Skeleton King start attacking Roshan to draw his aggro. This lets Ursa start attacking Roshan and slowly build up his Fury Swipe damage (which stacks damage per hit on targets, which is why he is such a good Roshan killer in general). Once Skeleton King gets low, have him move away so that Roshan starts attacking Ursa instead, pop the salve to heal, and go back in. Do the same thing if Ursa gets low on HP. If you do this correctly, you should be able to kill Roshan and grab the Aegis.

Tips to make this process even faster:

• You can actually forcibly move your hero after you pick, even if the Hero selection page is still up (manually click on the minimap, you should be able to purchase items as well but I'm not 100% on this part). This saves you some time heading there.
• Buy Boots of Speed. Considering you're getting +200 from Roshan, this is viable, although not necessarily recommended.

An alternative strategy is to use Lich with Ursa and cast Frost Armor, although this is slightly slower.

Be forewarned, however: If the opposing team sees you pick Ursa and SK in rapid succession, they probably know what you are up to and will head to Roshan in the beginning of the game as well. Considering you won't have a stun or a slow if you went Fury Swipes/Aura, this makes both of you easy targets for an early first blood.

Finally, in case you were wondering: Roshan is supposed to spawn every 10 minutes since he was last killed. The update addressed the issue where if you killed him before the timer started, it would actually take 20 minutes to spawn him after his first death.

To add more info on this:

since its not "fixed" yet (not sure if its a bug [see wc3 as reference]):

you can run immediately after picking your hero enabling you to rush to roshan even earlier. (you can see your health at the top so u can even try to do him if you are good)

Well. The easiest way to do it is having ursa in your team. Fury swipes attacks stack and stack and stack which would get you the roshan kill as fast as possible. The only problem remains to hang on enough in there with ursa until you kill Roshan. This means that if you go with your mates and you have someone to do some tanking for you, you should have no problem in killing him.

In dota 1 the easiest way was to go ursa/syllabear and summon the bear to do the tanking. I don't know if Syllabear is yet implemented in dota 2 but if not it will probably be soon implemented. Even so, tanking can be improved with some armor bonus skills such as lich's frost armor, maybe a bassilius etc. Also you should consider leaving the last hit to only one of your team's players as in early game some heroes starting with lvl 5 and a half can just turn the game by themselves. Vampiric aura from LeoriC(Skeleton king) is also a viable option but it practically doesn't help one bit in killing it faster. So maybe go for some attack speed aura as 90% of the damage will be done by ursa's fury swipes so just try to land them as fast as you can.

Ok. Now that it's settled that it's very easy to do this with ursa and kinda any random tank(as item maybe go gloves of speed on ursa),let's think if it's possible to do so with any other heroes.

Thinking

No. There you have it.

Without ursa I don't think it's possible to kill it before the creeps spawn, but this is an arguably +- 10-20 seconds marge of error.

In the "there is a chance to do it" category, pretty much any heroes that you can solo roshan at lvl 7 would increase your chances. These would be:Venomancer(Plague Wards)/Enigma(with rexx only be4 creeps spawn as you need to kill his pet to create minions as there aren't any minions be4 the game starts ofc)/Furion(Prophet-just spawn those trees) etc.

Also, if you play a semi-pro cough like game, probably you will need some smoke of deceit to get to Roshan undetected. Otherwise it's just gonna be sad.

Hope this was kinda helpful in helping you make an idea, if you need any more suggestions just ask(anything dota-related).

This being said, a more patiently written answer you will find here but it's from dota 1 so basically 99% of the stuff in there applies to dota 2 as well.