It is logical to think that it is in fact Hopper's sister who screams and Hopper himself who yells "Come Here".
In a later scene, when we see Hopper laying on the ground wounded, he is near the innermost transition bubble of even slower time. In this scene, we see Hopper's sister just inside the innermost bubble. She has the appearance of a scream on her face, and because she is just within the time transition, one can imagine that she has just entered in relation to Hopper's time.
Also, when Hopper first enters the cave and later when the teens enter, we see that the time outside the cave transitions roughly a year every few seconds or so. So even though Hopper's sister has been missing for years, in her timeline she may have only been down in the cave for a minute or two. We can infer a storyline for the sister in that, her and her parents wander into the cave, the parents are attacked and knocked out by the prehistoric savages, Hopper's sister is escaping through the dark cave when suddenly she happens upon her fully grown brother. Not recognizing him, she screams, he yells'come here', The teens hear this in the cave. Hopper's sister runs away from him and he gives chase. Hopper is attacked by savages and is wounded which gives his sister time to run away and into the next bubble of time compression, where a wounded Hopper then finds her with no way to rescue her without dooming himself in the process.