Trying to create a group expenses workbook

Hi there! I'm brand new on this forum and I have a very simple (or at least I think it is) problem..

I want to create a group expenses workbook and have been doing so for the last hour or so. The aspects that I want I can't seem to program myself though.
I'd like to be able to have the option of splitting the new expense evenly between the 5 participants. And if 'true' a cell will automatically populate with the value. However I would still like the option to do this manually or 'override' it if you like. I'm thinking the best way to do this is to have a true or false box first, then a rule like 'if true, = expense/5'

I've attached an image of what I currently have, maybe an 'if' statement? Although it just seems to return the 'NAME?' for me as I really am not experienced with excel.

Jack