Civilian spacecraft are fairly common; they're expensive, to be sure, and
if you're just going from point a to point b, it makes more sense just to book
passage on something, but many folks now make their living in space, so
transport is required.
Most of the non-aligned worlds have a less technologically advanced
society, but mainly what constitutes membership in the Big 5 is the size of
the government in question. If it's just one planet and maybe a sister world
or two, that's not enough to qualify.
It's based very much on the security council of the UN in that respect.