Just like the question indicates — if Fenrir Greyback didn't have a Dark Mark, how could he get through a barrier that required a Dark Mark to pass through it?
We see in HBP that the barrier that separated the Astronomy tower (where Dumbledore was to be murdered) and the battle required a Dark Mark to pass through.
"[...] and I'll bet you had to have a Dark Mark to get through that barrier [...]" -Harry Potter and the Half-Blood Prince, Chapter 29, The Phoenix Lament, pg. 621
We also know that Fenrir Greyback was present at the murder of Dumbledore.
"Is that you, Fenrir?" asked Dumbledore.
"That's right," rasped the other. -Harry Potter and the Half-Blood Prince, Chapter 27, The Lightning-Struck Tower, pg. 593
But we learn in DH that Greyback does not have the Dark Mark.
"I haven't got — they say he's using the Malfoys' place as a base. We'll take the boy there."
Harry thought he knew why Greyback was not calling Voldemort. The werewolf might be allowed to wear Death Eater robes when they wanted to use him, but only Voldemort's inner circle were branded with the Dark Mark: Greyback had not been granted this highest honor. -Harry Potter and the Deathly Hallows, Chapter 23, Malfoy Manor
So, if Greyback didn't have a Dark Mark, how could he get through a barrier that required a Dark Mark to pass through it?