Donation Method - Ask a bot for a Spare Key!
There is a Steam Community bot that people donate extra DotA 2 keys to. You can add the bot as a friend and send him the private message "give dota" to get a DotA 2 key sent to you. You can donate any extra keys you end up with afterwards.
New Method - Buy the access to Dota 2 Beta
Valve gives now the possibility to buy directly an early access bundle through Dota 2 Store. The bundle contains the beta access key and some collectibles.
Old method - Join contests/giveaways
Beta Giveaway Contests (by Dota 2 communities)
One legal and legit way, is to participate to one of many contests available and hope to win. The official community of Dota 2 is PlayDota. I suggest you to register to to that site, because since the launch of Dota 2 Beta it was the main beta key distributor, through random beta keys giveaway (hundreds if not thousands key already distributed).
There are many other places around to check and it has no much sense list all sites here. Just check PlayDota forum and Dota 2 Reddit Channel, that are the two main Dota 2 international community sites at the moment. If a new giveway starts, it is usually advertised there.
SteamGifts is also an interesting community driven gift website (that support Steam logon) that often has Dota 2 keys.
Beta Tester Selection (by Valve)
If you don't have time, skill or attitude for a contest, you can try to install Steam and fill this survey.
It is an hardware survey followed by a simple questionnaire that will be used by Valve to target potential beta testers. Identified subjects will receive the beta access and 2 additional keys to invite some friends.
Other sources
Be aware of scammers sites or people that try to sell you fake keys, there are many around. Also buying valid keys infringe Steam rules, so just avoid it.
Yes, Bane Nightmare's skill can be used to deny allies.
Nightmare can damage allies (and Bane), and can thus be used to deny hero kills.
reference
Regarding the damage required to wake up the target can be any type: AoE, target skill or auto-attack, even Radiance can do it. This is the reason why playing Bane within low pub teams can be difficult sometimes.
Transferring the Nightmare effect to the attacker only works with auto-attack instead.
Best Answer
You can mute individual players in the Score Board via the small speaker icon next to the player name.
There is no specific option to disable enemy chat by default.