Yes, Aberforth Dumbledore was a part of the original Order of the Phoenix, which was created during the First Wizarding War in opposition of Lord Voldemort and his Death Eaters. However, Aberforth was not an active member of the revived Order of the Phoenix, which was led by Albus Dumbledore again and consisted of Remus Lupin, Sirius Black, the Weasleys, Kingsley Shacklebolt, Nymphadora Tonks, and Mad-Eye Moody. He did help them towards the end as he helped the trio enter Hogwarts through a secret passageway and also fought in the Battle of Hogwarts.