Hiya Kelly! I'm Mal! Excited to have you joining us with Flash!
The most relevant to him would be:
Kitty Pryde (616): She's from sort of towards the end of X-Men Gold s she's already had her time with the Guardians where she worked with Flash. But I'm not sure on the timing on your end so it might just be a she knows him and not the other way around kind of thing.
Peter B Parker (sv): From spider-verse, he'd know about Flash but not this one so you know expect a lot of narrowed eyes and stuff.
Tony Stark (avac): he's from Avengers Academy and he's young and cute and not the old version at all. Flash was at the Academy too, so you know he'll know a version of him, but obviously not this one.
and then maybe America Chavez (616): But only from like a we're from the same world kind of thing. Have they met before? Maybe briefly during Civil War 2 when the Guardians came down on Carol's behalf? IDK!
After these four though I've got the others who won't know anything which are
Barbara Gordon (dcu) : Batgirl and sort of newish, but she's settling in, she'll be opening a library with Remus and Natasha and she's doing her best to keep her batfam together and not killers.
Klaus Hargreeves (ua) : He's just pleased that he's got two of his brother's here and that he feels partially responsible for making that happen, cause you know he's been maintaining sobriety on the off chance that has something to do with getting people here. He runs a bodega with Ben out of his house.