What's the relationship between Hrothgar and his retainers or vassals in Beowulf?

Hrothgar is the King.  His people are "retained" through loyalty and love.  In Anglo-Saxon times, loyalty and family ties meant everything.  Hence, Grendel is such a monster mainly because he is descended of Cain, who murdered his own brother.  Crimes against family, blood, and loyalty oaths were considered unforgiveable.

So, anyone fighting to protect Hrothgar would have received the pledge that the King would take care of him as Hrothgar expected the warrior to protect the King.  As payment for a warrior's loyalty, Hrothgar often gave rings or other trinkets of wealth.  This is how the kings of the time period came to be known as "the ring-givers".   The bond can only be honorably broken through death.