When the Order of the Phoenix was formed to fight off the evil caused by Lord Voldemort in the First Wizarding War, Remus had been one of the youngest Order members along with his friends.
The four friends had come to know of the Order via Dumbledore on their last day of the final term at Hogwarts. Dumbledore had called a few selective students including the Marauders and privately conveyed the Order's mission to them, giving them time to think it over thoroughly.
James and Sirius had agreed to join on the spot but Remus and Peter had taken their time, evaluating the pros and cons thoroughly.
However, Remus was also most probably the first werewolf who joined the Order. Later on he worked on getting more members from the wizarding minorities on their side.