Jaime here! We've been chatting on AIM about so much stuff, I don't think we ever talked about how our characters would get on :-p
Justin - she'd totally know him, terrible flirt that he is, plus since she used to date his best mate, she would have had to deal with him too - Justin & Ernie are a package deal!
Percy - she may have met him through the Ministry
Alicia - probably not too familiar, unless Siobhan was into quidditch much
Agatha - may or may not have met... but they will most definitely do so. Aggie's the Auror on Fenrir's case and is the one most likely to save Siobhan's arse ;)