I think it depends a lot on the phase the start up founders found themselves in. If the concept is concrete enough and the road map is more or less finalized, it might be a good idea to talk to your trusted friends and family members.
It's true that the initial road map the founders decide, keeps on changing as the idea evolves into an application and the further development is done based on the feedback stream obtained.
In my opinion the founders should keep themselves open to the idea of incorporating a few more people who love working in start up situations.
These people can also be a part of the initial team once the start up gets funding, only if one is looking for.