CoinJoin is a special kind of bitcoin transaction which aims to improve privacy. A coinjoin transaction is one where multiple people have agreed to form a single transaction where some of the the outputs have the same value. A casual observer of the blockchain cannot tell which output belongs to whom. CoinJoin transactions work today, and they've worked since the first day of Bitcoin. They are indistinguishable from normal transactions and thus cannot be blocked or inhibited except to the extent that any other Bitcoin transaction could be blocked.
This type of transaction was first described in a post by gmaxwell.
An example 2-party coinjoin transaction. https://chain.localbitcoins.com/tx/c38aac9910f327700e0f199972eed8ea7c6b1920e965f9cb48a92973e7325046 The outputs to addresses 1MUzngtNnrQRXRqqRTeDmpULW8X1aaGWeR and 1Fufjpf9RM2aQsGedhSpbSCGRHrmLMJ7yY are coinjoined.