I have a fairly large collection of people that Syd is probably going to have plottiness with!
I will start with my Georgia kids since she would have essentially grown up with them before she moved.
Ed (likeatree) - Matt's BFF. Ed doesn't really like her, because she broke Matt's heart. SORRY! Claudia (allthingsnice) - She chased after Matt for years and she considers him one of her BEST friends. Combine those two things and she really, really dislikes Syd. Partly from jealousy when they were younger and partly from Matt getting hurt. I can see some good snarking as a result. Lexi (sweetalexa) - Her big brother is good friends with Matt and she's kind of like a little sister. Gracie (talks2animals) - Ditto. Her brother is friends with Matt, she's like a little sister to him. Rissa (bellwatling) - She moved away to Chicago when she was 17 and had her senior year up there but now she's back in Atlanta for college. Ben (gentleben) - Matt's big brother. He's protective, but he liked Syd well enough and he doesn't hold a grudge like Ed and Claudia do. He'll be wary of her being back in Matt's life, but he won't be mean.
FROM NEW YORK!!! Maddie (briseisoftroy) - She's Matt's love interest, but also she grew up in New York, so there is a chance that they met when Syd was in NYC. I actually think she will like and respect Syd a lot, so that could make things interesting since she's now realizing she's in love with Matt. Chandler (corusrogue) - He also grew up in NYC, so they could have met when she lived there. He's been in London for school for the last couple of years, so if she's been traveling, they could have bumped into each other there, too.
FROM LOS ANGELES!!! You didn't specify where in California she lived, but if she was in LA then, she could know any of my kids who grew up there: Ollie (stupidbirds), Cyn (tinyblondone), Ellie (elluna), Livvy (justlivvy).
And... that's pretty much who I have that she is likely to meet/know. :)