I think the difficulty in joining the rangers would have a lot to do with when and where you were. The rangers of the early seasons would be near impossible to join. They operated in secret, revealing themselves to no one, secretly recruiting contacts and members while trying to avoid the notice of the Shadows.
By third season, the secrecy has been lifted somewhat. They have training camps on alien worlds, and the Shadows have become aware of them, so they are free then to operate a bit more in the open. People can know they exist, and they can recruit a bit more openly, but they still need to hide the organization's growth and that they are preparing for the Shadow's imminent return.
Once the White Star fleet is committed to the war and they have the first big battle against the Shadows, they can start putting up posters and opening recruiting offices. this is even more true after the formation of the ISA, when the rangers become the de facto military of the Alliance.