Graziano is a friend of Antonio and Bassanio. He's also quite the party animal. When Bassanio travels to Belmont to win Portia, Graziano wants to tag along, but Bassanio warns him to behave himself: "But hear thee, Graziano; / Thou art too wild, too rude and bold of voice" (2.3.5).
Apparently Graziano pulls himself together in Belmont, because he winds up getting hitched to Portia's sidekick, Nerissa, during the visit. If you're thinking, "Gee, Graziano's actions seem to parallel Bassanio's," you're absolutely right. Still, you'll be needing some more specific evidence to back this up, so pay attention. When Bassanio gets engaged to Portia, Graziano immediately announces his plans to wed Nerissa:
My eyes, my lord, can look as swift as yours:
You saw the mistress, I beheld the maid;
You loved, I loved for intermission.
No more pertains to me, my lord, than you. (3.2.2)
In other words, while Bassanio was busy falling for Portia, Graziano had his "eye" on Portia's woman in waiting. Notice the parallelism in the second and third lines of this passage? Parallelism is the similarity of structure in a pair or series of related words, phrases, or clauses: "You saw" / "I beheld" ; "You loved" / "I loved." This nifty little technique emphasizes the fact that Graziano's engagement to Nerissa mirrors, or parallels, Bassanio's engagement to Portia.
So why does Shakespeare orchestrate this double wedding? Well, it seems like Graziano and Nerissa's hookup says a lot about Bassanio's relationship with Portia and the nature of love and marriage in general. When Graziano makes his big announcement and admits that Nerissa only agreed to marry him if Bassanio won the casket contest, the couples' abrupt declaration of "love" doesn't exactly seem very genuine. Same goes for Bassanio and Portia, whose marriage is fuelled, in the beginning, by Bassanio's desire for Portia's money.
Graziano's behavior seems to copycat Bassanio's later in the play, too. Bassanio gives away Portia's ring after the trial, and Graziano gives away Nerissa's ring (4.2). Read more about this in "Symbols."