Here is the video:
It depicts Sam falling off a cliff and Frodo following after. We notice two elves approaching them. What business did these elves have at the black gate?
[Two Easterling soldiers see streams of dust coming down the cliff made by Sam and Frodo. They move away from the troop to investigate. Frodo reaches Sam who is stuck in the scree. As the Easterlings move closer and closer, Frodo throws his Elvish cloak over himself and Sam. The soldiers are now directly in front of Sam, but their eyes see nothing but rock. Frodo and Sam peer from underneath the cloak. The soldiers soon leave and after a moment, Frodo throws back the cloak and pulls Sam out.]
As to the purpose of their being at the Black Gate, Sauron is preparing for war and massing his troops.
MADRIL: Our scouts report Saruman has attacked Rohan. Théoden’s people have fled to Helm’s Deep. [Points at the map] But we must look to our own borders. Faramir, Orcs are on the move. Sauron is marshalling an army. Easterlings and Southrons are passing through the Black Gate.
I believe they were Haradrim, not elves. The Haradrim were the Southrons and were a people allied to Sauron, many of which had come north to fight. (Another troop of Haradrim were the people ambushed a bit later in the movie by Faramir. (The scene outside the Black Gate does not appear in the book.)