Theo's work colleague and mate, so there's that. Despite her laziness, he prob makes her work quite a bit. :P But he is happy someone else is all yay DE! He's know her since they were 11 so they'd know each other quite well and we discussed a lot of this.
Gwen's a year younger, so she'd know her from Hogwarts. Gwen will like her bitchy nature most of the time, and I imagine they'd get on well enough.
Mandy went to Hogwarts with her, so she will know her from that. She doesn't work at the Ministry, and isn't overly chatty, but will be nice to her.
Megan's apprehensive around Slytherins and I don't imagine they'd have seen each other much since Hogwarts, but she's welcome to tease and make fun of Megan ;)
Finally, not sure Marcus will know her, though they're bound to have some friends in common. P.S. She's Slytherin '98 ;)