Essential roles for web application team

Question

Some friends of mine came up with an idea for a web application which we (so far) think could be great. I made the analysis and all the early stages of the development process and I'm about to start the coding. I'm talking about something that is barely a mid-level project, so I consider one developer (myself) should be enough.

The thing is that we are trying to assign roles to each one of us so we can be focused on our duties and have clear our responsibilities within the team. We are a crew of four people, three of us (my friends) are business people who would do the marketing, customer relationship, management and accounting stuff and I'm basically the developer. I have in mind to get them involved into the development process by giving them documentation to write and use them as testers, all of that besides the management duties they have.

Perhaps someone out there have been in the same situation, so I would appreciate if the experience is shared so we can effectively give ourselves positions in the project based on what I explained above. Which are the essential roles or the optimal team layout so the idea can be developed successfully? The question is not strictly about programming, but it's related to build a software entrepreneurship beyond the code, that is something that I'm sure plenty of us are looking.

Any help is really appreciated! Regards.