Presumably the caves in the limestone cliffs around Engedi were deep enough for a group of men to hide in the back without being seen. Possibly, with Saul coming in from outside where there is light, his eyes would also take time to adjust to the darkness.
3 And he came to the sheepfolds by the way, where there was a cave, and Saul went in to relieve himself. Now David and his men were sitting in the innermost parts of the cave.