There are better ways than using email, that’s for sure. You can find a nice review of various collaborative writing tools on LaTeX wikibook: SpanDeX and ShareLaTeX allow for only one collaborator per project within their “free” licenses. WriteLaTeX is like EtherPad for
Making a poster with Inkscape (+template)
There is a very nice template for a scientific poster made by Felix Breuer using Inkscape, shared under Creative Commons Attribution-ShareAlike 3.0 Unported License. For many purposes it is maybe too fancy and you could find a square-panel layout more