Billy Bathsalt: Same as on the high seas.  Captain makes and enforces the rules.

With the notable change that the captain's masters, admirals Momentum and Energy, are VASTLY more pitiless than the cruelest sea on earth.

Until the far fully-industrialized-space future, there is no rescue. There are no islands. No coasting and waiting if you can't make the required delta-v.

There have been more than a few scifi stories that are variations on the theme of, "you stow away? You get spaced, no matter how sympathetic we are, because we cannot reach our destination with the added unexpected mass."