Simple answer: they're not. That's only in the movie, which bears only a VERY superficial resemblance to the epic poem. More complex answer: since Grendel is descended from Cain, and all men are descended from Adam and Eve, they are distantly related. This is part of what makes Grendel such an interesting character; he's not an inhuman monster, not an animal. He's a demonic, monstrous human.
Beowulf travels to Hrothgars mead hall in order to kill a demon that has been terrorizing the people who come to the hall.
Beowulf's father took refuge in Hrothgar's land after a messy murder back at home. He eventually married Hrothgar's daughter, and then came baby Beowulf. So long story short Hrothgar is Beowulf's grandfather.
'Beowulf' is the epic story that tells us the tale of hero Beowulf, the strongest and bravest of them all and how he defeats monsters and becomes a king. Among several characters we will also meet Hygelac, his uncle and king of the Geats, the one who opens his kingdom's doors and his heart to Beowulf.
Beowulf owes a family debt to Hrothgar because Hrothgar's father helped settle a feud and make peace with Beowulf's father, Ecgtheow. Beowulf's father was indebted to Hrothgar's father for his assistance, which Beowulf sees as a reason to repay the debt by helping Hrothgar against the monster, Grendel.
Hrothgar, king of the Danes, is a prime example of a great king. He also foreshadows Beowulf`s death in the Hall of Heorot, the story's most important speech. See the related links section below for an analysis of the speech.
Edgetho and Hrothgar are not friends. In the epic poem "Beowulf," Edgetho is Beowulf's father and Hrothgar is the king of the Danes whom Beowulf helps by defeating the monster Grendel. There is no indication of friendship between Edgetho and Hrothgar in the poem.
Beowulf and his men travel to Hrothgar's kingdom to kill Grendel.
Beowulf asks Hrothgar to send his armor to Hygelac if he is unsuccessful in defeating Grendel.
It takes Beowulf and his men five days to reach Hrothgar.
Beowulf first fights Grendel, a strong monster. Beowulf finds his weapons do no work against Grendel. He then fights Grendel with his hands. Beowulf rips off Grendel's arm and he runs away.
Hrothgar leaves Beowulf in charge of Heorot because Beowulf successfully defeated Grendel and his mother, ensuring the safety of the hall. Hrothgar trusts Beowulf's leadership capabilities and valor in defending the Danes from further threats.
Hrothgar tells Beowulf to not let pride get in the way of being an effective king. However, Beowulf does not listen to his advice. Beowulf's hubris becomes his downfall, which Hrothgar foreshadowed in his speech in the Hall of Heorot."Choose, dear Beowulf, the better part, eternal rewards. Do not give way to pride. For a brief while your strength is in bloom but it fades quickly..." (1759-1762)For more on Hrothgar's important speech to Beowulf, see the related links section below.