In Chapter 36 of Order of the Phoenix, Dumbledore makes a portkey to send Harry back to Hogwarts just after the duel with Lord V.
"I Shall explain everything," repeated Dumbledore, "when Harry is back at school."
He walked away from the pool to the place where the golden wizard's head lay on the floor. He pointed his wand at it and muttered, "Portus." The head glowed blue and trembled noisily against the wooden floor for a few seconds, then became still once more.
"Now you see here, Dumbledore!" said Fudge, as Dumbledore picked up the head and walked back to Harry carrying it.
Order of the Phoenix, Chapter 36 - Page 818 (scholastic 2003)
What amazes me is that Dumbledore then picks it up and isn't carried back to Hogwarts himself. Then, Harry is transported back to Hogwars immediately upon touching the statue head - no delay whatsoever.
We do know objects can be charmed to distinguish between the skin of one person and another (such as the golden snitch and wands for that matter) In the case of wands we don't really know how the wand distinguishes its master from other witches and wizards, but we know it does. In the case of the snitch however, it is touch sensitive and is created to distinguish which seeker caught it first. This of course, works after the seeker has actually touched it once in the first place. I simply site these as examples of magical objects being able to tell one person from another.
Can a portkey be made to be used by a single, specific person so that even if others touch it, it will not act as a portkey?