In Solo: A Star Wars Story, we learn that Enfys Nest and the supposed "marauders" aren't actually self-serving, but rather they seek to aid the oppressed and downtrodden, such as those on Savareen. I've seen the film twice so far and it seemed to me like Enfys Nest and company were just some random do-gooders. However I've seen several people offhandedly remark things like "that was neat how we saw the origin of the Rebel Alliance™ in Solo: A Star Wars Story" or "I thought A New Hope was Han's first time helping the Rebellion™, but I guess he did it first in Solo: A Star Wars Story". This all leads me to ask, were the "marauders" actually affiliated in any way with the Rebel Alliance™ at the time of their appearance in Solo: A Star Wars Story?
In current canon, the Rebel Alliance was officially formed in 2 BBY (two years before the events of A New Hope) in a speech by Mon Mothma, in the Star Wars: Rebels episode "Secret Cargo". That formally brought together many existing rebel cells such as the crew of the Ghost and Phoenix Squadron (from Rebels) but some groups weren't invited/were disowned, such as Saw Gerrera's (as seen in Rogue One and Rebels again).
Solo's scenes with Enfys Nest take place eight years prior to that, in 10 BBY, so her marauders definitely weren't associated with the Rebel Alliance at the time, since it didn't exist. They also seemed to be primarily focused on revenge against Crimson Dawn and other crime syndicates, rather than the Empire.
Enfys was closely allied with the Alliance to Restore the Republic. It's not clear whether there's anyone that she ultimately reports to (other than her own sweet self), but in the closing pages of the film's canon novelisation we see her dropping the coaxium off with Saw Gerrera and his group of partisans. It's noteworthy that this is a mighty prize and that at this point in time, Saw was also strongly aligned with the Rebellion proper so it's likely that she was either told where to take it by Mon Mothma or just decided off her own bat that he was best equipped to use the coaxium to hurt the Empire.
And it was probably for the best that she hid her identity even from her allies; it wouldn’t do for them to start doubting her because of her age.
He actually came alone. Brave, Enfys thought.
“Saw Gerrera,” she said, her modulator disguising her soft voice.
“Enfys Nest,” he said, glowering. “I hope you have good news for me.” He sounded as if he didn’t believe she ever could.
“Refined coaxium, enough to power a fleet,” she said, gesturing behind her. “I trust you will put it to good use.”
The key is in the name 'Rebel Alliance'. Although Enfys Nest are rebelling against the empire, they are not the sole creators of the Rebel Alliance we see in A New Hope.
The Rebel Alliance is actually formed after the events of Solo and is made up of several Rebel factions with the overall aim to overthrow the empire. Enfys Nest is presumed to be part of this Alliance whilst there are also other Rebel factions that operate independantly from the Rebel Alliance e.g. Saw Gerreras faction known as the Partisans.