Jen & Justin Engaged: Yes! Yes! Yes!

Diners at the high-end, understated Blue Hill restaurant in New York City on Friday, Aug. 10, barely took notice of one of Hollywood’s power couples, actress Jennifer Aniston and actor-screenwriter Justin Theroux, quietly celebrating his 41st birthday side by side at an intimate corner table. There was no popping of a champagne cork. No raucous laughter or squeals of delight. Rather, the two, who had flown into town for the weekend from the Wilmington, N.C., set of Aniston’s latest film, We’re the Millers, “looked like a long-term couple in love having a romantic dinner,” says a patron. The duo had a soft conversation over white wine. “He looked very relaxed with her,” says another diner, Monica Diorello. “They were just enjoying each other’s company.”

Turns out the party was just getting started. After a 15-month courtship that spawned a constantly revving rumor mill-babies! breakups!-the two soon let the world in on their very, very happy secret. On Aug. 12 Theroux’s rep exclusively told PEOPLE that the actor “had an amazing birthday on Friday, receiving an extraordinary gift when his girlfriend, Jennifer Aniston, accepted his proposal of marriage.” While there are no details yet on the ring he gave her, “everyone is really excited for them,” says a member of their inner circle. “It’s amazing to see how happy Justin makes Jen, and everyone was always hoping that he would propose.” A Theroux pal adds of the question-popping: “He’s been dying to do it. His friends knew it was coming.”

After her heartfelt “yes,” Aniston, 43, and Theroux promptly rang up family and close pals to share their big news-including the actress’s father, actor John Aniston, 79, in L.A. “John was one of the first people Jen called,” says a source close to him. “He loves how happy Justin makes his daughter, and he more than anyone wants to see her get her happily-ever-after. He said Jen sounded ecstatic and happier than she’s ever been.” As for the Theroux clan, the actor’s sister Elizabeth says Aniston has already won them over. “They’re a wonderful, beautiful couple,” she says. “We’re so happy to welcome her to the family.”

Nearly inseparable since taking their romance public in June 2011, Aniston and Theroux first became friends while filming the comedy Wanderlust in 2010. Aniston, who had dated Vince Vaughn and John Mayer after her 2005 divorce from Brad Pitt, wasn’t looking to get serious again soon: “I’m really picky. When I feel it, I feel it,” she told PEOPLE in 2011. But Theroux was in a long-term relationship with stylist Heidi Bivens, 36, and “Jen would never be with a guy that has a girlfriend,” a source said. By May 2011, Theroux and Bivens were over, and Aniston started spending time with the sometimes-scruffy-bearded, witty New Yorker who has written blockbusters like Iron Man 2 and guest-starred on such critical darlings as Six Feet Under and Parks and Recreation.

Soon she introduced him as her boyfriend to her close-knit group of female friends, including Courteney Cox and Chelsea Handler-all quick to give Theroux their stamp of approval. “From the beginning, Jen’s friends thought that they were right for each other,” says a source. Despite his edgy hipster vibe, Theroux, who was raised in Washington, D.C., by his lawyer dad and journalist mom, proved a perfect match for the actress. “They share many great things, including the love for their jobs and the same low-key attitude about life,” says the member of her circle. “He treats her like gold, and they just click and complement each other.” Theroux told Extra last year, “I always go to bed thinking I’m the luckiest guy in the world.” And Aniston told an Aussie radio station her happiness level was a “10-plus.”

By summer 2011 the two were considering a permanent move to New York City, and Aniston bought adjoining apartments in the West Village. Yet the constant presence of paparazzi irked the couple, and they retreated back to L.A. to house hunt. In fact, the scrutiny has been one of the biggest hurdles for them. “It’s aggravating they can’t walk around New York and just be themselves,” says a Theroux source. “He struggles with her level of fame, but he’s not going to let it come in between them.” After she sold her Beverly Hills mansion, the two settled on an 8,500-sq.-ft. Bel Air estate, which they are remodeling while living in a rental. “They’re having a lot of fun creating their first home together,” says a source.

With Aniston working in Wilmington and Theroux shuttling between his fiancee’s set and L.A. and New York for work (he plans to direct the comedy Swear to God starring Will Ferrell and Steve Carell), wedding plans might have to wait-or not. “I wouldn’t be surprised if they eloped,” says his friend. Other pals bet on an intimate, relaxed ceremony, possibly a destination wedding (she does love the beach!). And after marriage, could babies be far behind? The couple haven’t hidden their affection for kids-Aniston recently visited with Robert Downey Jr.‘s newborn, and Theroux doted on the children of Aniston’s pals Cox and Laura Dern at a July 31 dinner in New York City while Aniston was on-set. “Jen loves babies,” adds a source. “They would be great at raising a family together.”

A joyful Aniston was spotted returning to Wilmington late on Aug. 12. “Jen has a wonderful life that has always been very complete, and this is just the icing on the cake,” says the insider. “She certainly didn’t need a ring…. She didn’t even know if she would get married again. It just must have felt right.”