- Men guarded the gates
- Men lived in Isengard
- Men collected Saruman's supplies
The keepers of the gates
They were Men.
'We wouldn't ask you to,' said Merry. 'We have had enough of Orcs ourselves to last a life-time. But there were many other folk in Isengard. Saruman kept enough wisdom not to trust his Orcs. He had Men to guard his gates: some of his most faithful servants, I suppose. Anyway they were favoured and got good provisions.'
The Lord of the Rings: The Two Towers, Book III, Chapter IX: Flotsam and Jetsam
Think about it: Wouldn't it have been strange for Saruman to keep Uruk-hai as his gate keepers, prior to fully revealing his traitorous plans? Gandalf would have immediately suspected Saruman of treachery if he had Orcs at his gates, not Men.
Who else stays at Isengard?
Not only Uruks and Saruman. Men, too, stay at Isengard.
Aragorn and his companions sat themselves down at one end of a long table, and the hobbits disappeared through one of the inner doors. 'Store-room in there, and above the woods, luckily,' said Pippin, as they came back laden with dishes, bowls, cups, knives, and food of various sorts.
'And you need not turn up your nose at the provender, Master Gimli,' said Merry. 'This is not orc-stuff, but man-food, as Treebeard calls it. Will you have wine or beer? There's a barrel inside there that's very passable. And this is first-rate salted pork. Or I can cut you some rashers of bacon and broil them, if you like. I am sorry there is no green stuff: the deliveries have been rather interrupted in the last few days! I cannot offer you anything to follow but butter and honey for your bread. Are you content?'
It was rather a large portion of food. Clearly the Men had taken up abode at Isengard to stockpile such a large number of "man-food". (also see the first quote in Matt Gutting's answer, referring to Isengard's houses)
Who collected Saruman's supplies?
Who indeed...? You guessed it -- Men!
Saruman had long taken an interest in the Shire - because Gandalf did, and be was suspicious of him; and
because (again in secret imitation of Gandalf) he had taken to the "Halflings' leaf," and needed supplies,
but in pride (having once scoffed at Gandalf's use of the weed) kept this as secret as he could.
Some while ago one of Saruman's most trusted servants (yet a ruffianly fellow, an outlaw driven from
Dunland, where many said that he had Orc-blood) had returned from the borders of the Shire, where he
had been negotiating for the purpose of "leaf" and other supplies. Saruman was beginning to store
Isengard against war. This man was now on his way back to continue the business, and to arrange for the
transport of many goods before autumn failed.
Unfinished Tales - Part III, Chapter IV, The Hunt for the Ring
Therefore, it would seem that Saruman entrusted Men, to be more specific; Dunlendish Men, with jobs in Isengard.