During Frealaf's reign in Rohan Saruman came offering him gifts and buttered him up, and all the Rohirrim seemed to like him. Then Beren the current Steward of Gondor let Saruman reside in Orthanc and gave him the keys to Orthanc (since Orthanc was still claimed under Gondor's rule at the time). Which we are told:
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'In this way Saruman began to behave as a lord of Men; for at first he held Isengard as a lieutenant of the Steward and warden of the tower. But Frealaf was as glad as Beren to have this so, and to know that Isengard was in the hands of a strong friend. A friend he long seemed, and maybe in the beginning he was in truth.'~Appendix A: The House of Eorl
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So, that's how Saruman gets control of Orthanc, I don't know if anyone else lived within Orthanc itself, or whether they lived in the Ring of Isengard. Though Saruman did have Men under his command, as Gandalf remarks that the Gate into Isengard was strongly guarded, and by Men who had been awaiting for him to come see Saruman.
Though this doesn't appear in the movies, Saruman did have other servants besides Orcs and Wormtongue, but it always seemed like the Pinnacle itself, Orthanc, was the abodement of only Saruman.