Jennifer Aniston and Justin Theroux threw a HUGE base at their Beverly Hills mansion last night, to celebrate their recent engagement, X17online can report exclusively.

Friends like Demi Moore, Ben Stiller, Lake Bell, Ellen DeGeneres and Portia di Rossi turned out for the big bash.

Guests arrived in casual attire, with gifts. The party started around 7pm and some of the revelers, like Demi, left around 11pm.

A DJ played alternative music ... not too loud ... while the party went on inside and rain poured down outside.

Jen and Justin were engaged in mid-August while Aniston was filming We're the Millers in North Carolina. The two dated for just over a year after meeting on the set of Wanderlust.

Justin Theroux took a break from riding his bike and decorating his home and made it to the voting booth on Tuesday afternoon, proudly displaying his "I Voted" sticker on the front of his denim jacket.

The 41-year-old actor/writer rocked his signature dark skinny jeans and boots, and while we definitely see why Jennifer Aniston is smitten with him, we have to ask ... does he own anything other than black and gray? Not like we mind, but when have you ever seen him in color (or white, for that matter)?

They're already running errands together!

Jennifer Aniston and Justin Theroux decided to spruce up their Bel Air mansion when they visited furniture stores Adesso and Blackman Cruz in Hollywood on Monday.

The newly engaged couple spent the weekend partying with fellow celebs as they met up with Drew Barrymore, Will Kopelman and others at the LACMA 2012 Art + Film Gala which honored Ed Ruscha and the late film director Stanley Kubrick Saturday night. Good to see these lovebirds off to a great start in their engagement!

Jennifer Aniston and Justin Theroux are engaged, a rep for the actress has confirmed. Theroux's rep added, "[Justin] had an amazing birthday on Friday, receiving an extraordinary gift when his girlfriend, Jennifer Aniston, accepted his proposal of marriage."

The two began dating in May of 2011 shortly after meeting on the set of the comedy Wanderlust. This will be Theroux's first marriage and Aniston's second -- she famously married Brad Pitt in July of 2000, but their marriage ended in 2005 when Brad became involved with Angelina Jolie.

Check out our favorite shots of the couple in the gallery below!

Justin Theroux has booked a new gig as director of Swear To God ... an upcoming comedy starring Will Ferrell and Steve Carell, according to Deadline.com.

Theroux will also rewrite the film's current script.

IMDB describes the movie as follows: "Ferrell will play a narcissistic hedge fund manager who thinks he has seen God."

Jennifer Aniston's beau also recently scripted Tropic Thunder and Zoolander 2.

Will Ferrell will begin filming Swear To God once he wraps the Anchorman sequel.

Three's company! Jennifer Aniston, beau Justin Theroux and pal Shirley MacLaine enjoyed a dinner date at Giorgio Baldi in Santa Monica.

The famous trio certainly made a splash while leaving the pricey restaurant ... sending flashbulbs into overdrive!

We love how Jennifer seems shy while making her way to the car, but then as soon as Justin's by her side, the sexy Friends actress is happy as can be.

Oh, to be a fly at their dinner table.

Fresh from their romantic European vacation, Jennifer Aniston flashed a huge diamond engagement ring on her right finger as she happily made her way through the LAX airport with beau Justin Theroux, on Monday afternoon.

The happy pair of one-year already wear matching gold "Jennifer" and "Justin" rings on their respective left ring fingers, but the huge rock Jen showed off appears to be brand new. She may be wearing it on her right finger just to trick us!

It's been reported that the couple were in Europe to scout out potential wedding locations in France, so Justin may have popped the question while they were on their trip!

While girlfriend Jennifer Aniston was getting ready for the tribute to Shirley MacLaine at the AFI Awards, her beau Justin Theroux hit up Lemonade in LA with some buddies for an afternoon lunch date.

Dressed in a killer biker jacket, Theroux looked ruggedly handsome while holding on to his refreshing drink.

Jen and Justin are currently working on a short film project together that will spoof their life in the media spotlight, according to Page Six. A source tells the site, “It is a parody of the paparazzi and tabloid interest in Jennifer’s life.”

The film, produced for Smartwater, will be part of a viral ad for the water brand. Production for the movie project begins next month.

Great to know these two have a wicked sense of humor!