So, there are a number of ways that games have tried to resolve this in the past, many of which are based around the systems involved. A few of the ones I can recall off the top of my head:

*) Everyone has to roll, and if anyone fails, you're plumb out of luck.
*) Everyone gets to roll, and if even one person succeeds, all is good.
*) One person rolls, and everyone else provides a bonus to that roll based on how much they're helping.
*) Everyone has to roll; you take the best result, but if someone fails, it applies a cost of some kind.
*) Everyone has to roll, and people can spend some of their successes to cover for people who fail.