Yeah, that's my thoughts as well.Well, she mentions that her dad thinks she's staying over at Amber's place that night. Which most likely means he dropped her off there or something. The usual way that thing works is the girls make their plans together to get their stories straight in case there's some checking up, so you know for damn sure Amber knew what was going down. Amber either followed her or was straight up invited.
I don't think it's probable Evelyn straight up invited her though. Not because I think Evelyn would be uncomfortable doing so, but Evelyn might suspect that a direct invitation might only spook Amber into not coming. Evelyn knows Amber isn't as comfortable with sex as she is, or at least Amber's response at lunch should be a strong clue.
However, making sure Amber knows when and where it's going down, teasing the event all day, and setting up circumstances where Amber could easily follow her... that I definitely could see.