If you want to maintain the friendship while doing business with each other then:
-First make sure everyone is ready to take up a role. Their role will be their responsibility, so it is better if you will give a role that fits everyone's expertise.
-Second, patience and fact checking is a must. Just because you are all friends, fights and arguments are not inevitable.
-Third is be professional. Business is business so everyone must take this one seriously.
-Last, take criticism as an advice and be honest to each other at all time. Stop going behind each other's back when sometjing went wrong. Face each other at the same time face the problem.
Grown ups are still childish sometimes but it can be fixed.