if you're just starting out, and pre-outside investment, my advice is don't worry too much or waste time/money on legal fees - concentrate on your core product/service and leave the legals until you agree to take on funding.
suggest all you need is a simple agreement between the founders that deals with:
1. the (fairly high) chance that people may leave (for whatever reason) and new people may join before you reach the next stage, and
2. what equity split the founders get (suggest equal unless there's a major difference in input e.g. one person full time, others weekends..), a common understanding of what's required to maintain this (i.e. effort/results required) and a simple procedure for what will happen if people's input doesn't match this (basically make sure everyone accepts that they don't get automatic entitlement to their share up-front - they have to work for it, and if they don't pull their weight then the others are entitled to ask for a review of the split)
if you have a situation where some co-founders are "conspiring" against another, there's going to be little point in that founder trying to stick around - something underlying will be seriously wrong and the startup's chances of success will be majorly scuppered - so i think thats a risk everyone has to take - it's down to each individual member to make themselves crucial, and if your co-founders turn out to be s then take your vested shares to date and move on quick.
of course change everything soon as (or just before) outside investment - raise enough to cater for paying your lawyer to sort this when finalising the deal!