I assumed a world (or group of worlds) had to be members of the Federation in order for their populace to join Starfleet; other than officer exchanges with allies of the Federation, of course.
The it struck me that Nog is a Ferengi, yet he joined Starfleet. I am pretty sure that the Ferengi Alliance isn't officially an ally of the Federation, let alone a member (they seem to enjoy the status of neutral parties, for maximal business opportunities).
Is this correct and if so is this an exception to the rule or can people from any planet join Starfleet?