The simplest method would be to agree between you that a nominated individual will automatically be sent a copy of every email that is started by any of you. Then, when someone wants to respond to something all he or she has to do is click on the 'reply all' button.

That way the individual concerned will accumulate a complete archive of all email exchanges and everyone else in an individual exchange of comments will have a record of what everyone else said.

Alternatively - and this is probably better if the discussions tend to be wordy - you could type up comments on a Word document and send it around using this free facility which is used by millions of individuals and businesses. It works beautifully, and is very secure. Each person gets access to the same discussion document, and can add comments for everyone else to see.