Jonathan is a friend of Brent's from school. Brent gives him a ride to the party, which is hosted by a friend of Jonathan's.
When Jonathan comes out and gets in Brent's car, he explains that his father took his car because he forgot to pay his insurance bill. Then he tells Brent to relax when Brent worries that he was not explicitly invited to the party. Jonathan tells him that since he, Jonathan, is friends with both of them, he's welcome.
Brent sees the party as a chance to get in with a good crowd and make friends at his new school. Ultimately he fails and, in an attempt to kill himself, kills someone else. This is what puts him on the path to place whirligigs in four different places around the country as penance for the woman's death.