Of course she came on to him and then lied about it... She never really went backpacking across Western Europe. :) This whole thing also hinges on some subtle acting by Aniston.
Joey: It’s this story I came up with, very romantic. I swear any woman that hears it, they’re like putty. [...] Now you’re gonna want to have sex with me when you hear it, but you have to remember it's just a story. [...] Years ago, when I was backpacking across Western Europe...
The chain of events is this: Joey comes up with the Europe story. He tells it to a girl to sleep with her; she tells Rachel she slept with a guy after he told her this story. Joey then tells it to Ross, who hadn't had sex for a while, with the understanding that Ross should tell it in the 1st person.
And this is exactly what Rachel does. You can see her making her mind up after the awkward silence when she and Ross admit they would've slept together after Monica's engagement. She knows Ross is not exactly a spontaneous guy and would need some convincing, so she says:
Ross, did I ever tell you about the time I went backpacking through Western Europe?
It's highly unlikely that a spoiled rich girl like Rachel ever did that, and even if she did, that it fit exactly into Joey's story. She simply lies that she did, and has no reason to make up such an extravagant lie that's easy to pick apart just to make conversation. So she's secure in the fact that no one would know this story, but then:
Monica: You used the Europe story!
Chandler: That’s the magic story you use when you wanna have sex!
Rachel: How do you know about that story?
Rachel sounds disappointed when she says that, knowing she was caught in a lie. Her first reaction isn't in the line of 'it wasn't a come on' but rather 'damn, you know the story too': Now they know she only told this made-up story to seduce Ross.