Sorry, as a result of my internet going down yesterday, this post is a day late! I used my main textbook to write this one, which is what the reference is for below.
Joint enterprise has come into the spotlight in recent years, as offences of violence involving groups of young individuals has increased. The crime is often done with a weapon. So, in the circumstances that you are in a gang or group of individuals, and you know that one of your friends/companions is carrying a knife on him and that he 'might' use it against another individual - you can very well be guilty of an offence.
All participants are usually found to be guilty in joint enterprise cases. Joint enterprise cases are often where the parties are doing the same act together (i.e. beating someone up, to put it bluntly) or having a plan to commit a crime. When someone exceeds this plan, i.e. takes it one step further, the other party members will be liable under something called parasitic accessorial liability. (Smith & Hogan, 13th edition, 2011, OUP). Liability as joint principles arises where each has done his own act which is a part of the actus reus.
An example would be the case of R v Powell - Powell (the defendant) (P) and another had a plan to buy drugs. P knew the other had a gun, and knew it might be used to cause serious harm or kill, and as such, his appeal to the House of Lords was dismissed and he was found guilty to have committed a crime (murder) after the other used his gun to kill the drug dealer.