Jimmy Kimmel and Molly McNearney's July 13 wedding in Ojai, Calif., was a star-studded affair. Celebrity guests included Matt Damon, Emily Blunt, John Krasinski, Ellen DeGeneres, Portia de Rossi, Ben Affleck, Jennifer Garner, Dax Shepard, Kristen Bell, Johnny Knoxville, Kelly Ripa, Mark Consuelos, Gabourey Sidibe, JJ Abrams, Don Cheadle, Andy Richter and Jeff Ross -- not to mention future spouses Jennifer Aniston and Justin Theroux.

Before the ceremony began, however, the Wanderlust costars visited the Hacienda Shop at Ojai Valley Inn & Spa. "They were the nicest, sweetest people and were completely relaxed," a source tells Us Weekly. "They're very relaxed together and spent a lot of time sunbathing at the spa with all the other huge celebs."

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When the bride and groom swapped vows around 5:45 p.m., "Jen and Justin seemed very happy and in love," the source says of the actors, who got engaged in August 2012. "She had her hand on Justin's knee for most of the ceremony and was intently listening. At one point, she put her arm around Justin's neck and then put on her reading glasses and glanced at the wedding program."

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The source adds that Aniston, 44, "looked genuinely happy" for Kimmel, 45, and McNearney, 35, as they recited their vows. After the Jimmy Kimmel Live! host and the show's co-head writer were wed, Aniston chatted up her good pal Blunt, 30. "While Jen talked to Emily, Justin talked to Ben Affleck and John Krasinski," the source says.

Aniston and Theroux then rode a golf cart with DeGeneres and de Rossi to a cocktail reception down the hill. "Jen talked to Ellen and Portia at a tall bar table top for the majority of the cocktail hour," according to a source.

PHOTOS: Jennifer Aniston's Us Weekly coversJust before 7:30 p.m., Aniston and Theroux "leisurely strolled into the [reception] tent together with their arms wrapped tight around each other's lower backs," another insider reveals. "Jen rested her head on Justin's shoulder as they emerged from the quiet pathway."

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During the reception, Kimmel and McNearney's siblings toasted the newlyweds. The party ended around midnight, according to a source. Although the reception officially ended around midnight, many revelers continued to celebrate until around 2 a.m. at an afterparty held in the ballrooms overlooking the herb garden. The guests indulged in mini sliders and a variety of other late night food while dancing into the morning hours.

This article originally appeared on Usmagazine.com: Jennifer Aniston, Justin Theroux Attend Jimmy Kimmel's Wedding