A founders shareholder agreement, also known as SHA (in some cases in the U.S. referred as a stockholders' agreement) is THE key tool to use to get your startup team to commit to the shared venture as co-founders - to start making your startup more than just an idea or a dream. Use it as a tool and a foundation of your startup venture, to move it beyond ideation (-2) or concepting (-1) phase to commitment phase (0). It will have a big impact on your startup credibility and valuation.
Link to original source: http://www.startupcommons.org/founders-shareholder-agreement.html
A pre shareholder agreement is recommended for those entrepreneurs that are in -2 and -1 stages, in the middle of the process to build a team in which you have no shareholder agreement and you are looking for initial commitment from your potential team members.
Link to original source: http://www.startupcommons.org/pre-shareholder-agreement.html