It takes some fast talking to reassure the startled demon that no, they're not a joint team from Upstairs and Downstairs sent to take him out. They're not even associated with this universe's Upstairs and Downstairs.
"The different-universe part is likely true," puts in the humanoid information terminal who'd been striking a kung-fu pose at his side, "as I have no information on record about either of them. The rest of their story can neither be confirmed nor denied, as, again, I have no information on record about either of them."
She relaxes into a perky at-attention pose, which neither Crowley nor Aziraphale are fooled by. She has access to, among other things, all the martial-arts and self-defense knowledge in the universe. Could probably speed-bless the contents of one of the beer bottles along the bar wall in the time it takes to swing it at Crowley's forehead.
"So you're already in rebellion against the celestial structures of your world?" says Aziraphale hopefully, trying to steer the conversation back to their areas of agreement. "Taken your stand on the side of the human race?"
"Because that's us too," says Crowley. "Great fans of humans, us. Squidgy bits and all."
The other demon squirms. "I wouldn't say the entire human race," he admits. "Sure, Janet and I sort of worked our way up to wanting to destroy and rebuild the whole soul-judging system from scratch. But only because it turned out that's what it'll take to protect...those four over there."
He gestures to the gaggle of humans cowering behind the bar. Two are staring at Crowley and Aziraphale in the standard celestial awe/terror; a third, in a less-typical but still-understandable reaction, has her eyes fixed on Crowley's hips. The fourth, amazingly, has been distracted by a novelty napkin ring.
Leaning in toward Crowley for a conspiratorial whisper, the local demon adds, "The reason is friends!"
"W-well. Yes. Very good," says Aziraphale. They can still work with that, surely. "It would be to our mutual benefit -- the two of us, the two of you, and your friends -- if you joined us in the Pan-Universal Eternal and Corporeal Alliance to Defend the Best Interests of Humanity."
For the first time, one of the humans speaks. "That's what we're calling it? Yeesh, kind of a mouthful."
Aziraphale is officially flustered. "I'm sorry, did you have a better idea?"
"No, we did not," says another human, snapping out of her awe/terror to glare pointedly at her friend. She switches on a dime to beaming at the visitors, as charming as if they were invited guests to her interdimensional cocktail party. "Of course, we would love to help. Have you found any volunteers yet to bring the hors d'oeuvres?"