Shamir secret sharing doesn't meet all of your requirements. You can use it so that the password is shared over five people, and three of them are required to reconstruct the password. Also, at least one of you will need to know the password to split it up into secret shares in the first place. Also you need to generate the proper keys to know the bitcoin address, thus you need to have a password to use with brainwallet in the first place, and therefor it is obvious that someone will know the password. It can be split up and shared with five people with secret sharing schemes, but yeah one will need to have the ability to know the full thing because it is split up from the whole thing in the first place. SelfSovereignty gave the best solution I think.