(written from a Production point of view)
Aurelia was, according to the script of the Star Trek: The Animated Series first season episode "Yesteryear", a Federation member planet, and homeworld to the Aurelians. Neither Aurelia nor Aurelians were referred to in the final version of "Yesteryear".
According to Star Trek Maps, Aurelia was the second planet in the Xi Herculis system.
Aurelia was on the list of member planets in the Star Trek: Star Charts. (United Federation of Planets I)