The rapper, who has already shot scenes for the season finale, told Grenier that he was a fan of the show when they worked together.
"I actually met Eminem several months ago, interviewing him for a documentary we're making," Grenier told Popeater.
"He mentioned that he loved the show and I said, 'Do you want to be on it?' And he said, 'Yeah'. So I called [the show's creator] Doug Ellin right then. I said, 'Doug, how would you like Eminem to be on the show?' He said, 'I'd love it!'"
However, Grenier teased that his character Vince will not have a good relationship with Eminem.
"Watch what you ask for," he joked. "We don't even get to be buds. Doug wrote this whole thing - I can't mention what it is, but Eminem and Vince have a conflict. It's very interesting, you have no idea."
Entourage continues on Sundays at 10.30pm on HBO.