What is the significance of the guys on bikes who always seem to be following Scarlett Johansson in Under the Skin?
In the novel Under the Skin by Michel Faber, Scarlet Johansson's character is an alien on Earth, selecting prime human specimens for the delicatessen back home. She isn't on Earth alone in this task, and so the guys on the bikes are also aliens, who assist her with the collection of the human specimens. When she goes walkabout, her fellow alien biker comrades try to track her down, in fear of their true identities being discovered.
They are also aliens; kind of working as her overseers. There's one main guy on a bike; you meet him at the beginning of the movie and several times throughout. He cleans up after her, like when she kidnapped a guy at the beach and then the biker picked up the guy's tent, so no one would realize he was missing. The main biker guy also made her stand in the dark room and inspected her eyes about halfway through to make sure she wasn't forming sympathy towards humans like the other female alien at the beginning of the movie. When she eventually does start to have sympathy and runs away, the main biker along with several other bikers all set out to find her so that the humans won't find out she is an alien.
You can watch this for a more detailed explanation of the movie: