Fallout 76 allies are new additions to the game with the release of the Wastelanders expansion. Each Fallout 76 ally will serve as a companion to a limited degree: they don't follow you all around the world, but they live at and defend your camp, give you quests to complete, reveal their backstories through conversations, and you can romance them.
Finding and recruiting these new allies, Commander Sofia Daguerre and Beckett, require activating and completing a few multi-stage quests. Thankfully, you don't have to pinpoint each ally's exact location to begin their quests—just wandering into their general part of the world will trigger them.
Note: You'll need to be level 20 to begin these ally quests, and you should be prepared for quite a battle when recruiting both of these allies. You can also only have one companion living at your camp at a time, but you recruit both and swap between them without much fuss.
How to find Commander Sofia Daguerre
Commander Sofia Daguerre is an astronaut who recently crashed to earth. She's located in The Mire to the north-east section of the map, and wandering anywhere near that area will let you pick up her distress signal and begin the quest once you've reached level 20.
Daguerre's signal will lead you to East Ridge Lookout, where at the bottom of the cliff you'll find her in a bunker. She'll direct you to her crashed ship where you need to download some data. While you're waiting for the download, be prepared for an attack from some super mutants—I was level 43, and some of them were level 68, so expect a tough fight... or just run around in circles avoiding them like a coward until the download is complete, like I did. Then grab the data and run like hell.
After one more mission to locate a wandering robot in The Mire, you can return to Sofia and offer to let her stay at your camp. Build the USSA console at your camp—you'll find it on the build menu under the Allies tab—and she'll promptly show up and start living there, allowing you to access her additional quests.
How to find Beckett
To find Beckett you'll need to head down toward Ash Heap in the south-west area of the map for his quest to fire off. He's been imprisoned at a new location, Rollins Labor Camp, by the Blood Eagles, a new gang faction. There are lots—lots—of Blood Eagles at the camp, so get ready for a fight.
Talk to Beckett while he's still in jail and he'll ask you to find a key, which is back outside in one of the towers and will result in more Blood Eagles showing up. Once you've gotten the key and freed Beckett, he'll send you to another Blood Eagles camp even further south to collect his gear. Again, there are a buttload of Blood Eagles here, though Beckett's gear is around the back and you may be able to sneak in and nab it without alerting the Eagles.
Head to your camp and build Beckett's Bar—it's in the build menu under Allies. You won't be able to place it in your camp if Sofia's USSA console is already placed there, and vice-versa. Swapping between the allies just requires removing whichever Ally object has been placed and building the other one.
How to romance your Allies
If you've played other Fallout games you'll already be familiar with how romance works with your companions—you talk with them, complete personal quests, and unlock new dialogue options over time.
It's much the same with Fallout 76's allies: while you're at your camp, talk to an ally and see what quests they're offering, complete them, and unlock new conversations. Flirting will become an option once you've gotten to know them a bit better, and the deeper you go into their questlines the more opportunities to romance them become available.