Allison has a certain level of insecurity and low self-esteem, especially when it comes to impressing Ian. I can't recall if it's explains the actual reason for her low self-esteem, but perhaps it's to do with her being quite tall (I have a niece who is tall af an it always made her self-conscious cos guys felt weird about dating a girl taller then them), or feeling like she's not quite the prettiest or most interesting girl in the room? Or maybe it's just that she has been in love with Ian for so long yet somehow felt inadequate next to a sexy redhead like Gillian, or isn't sure how she measures up to his ex now that Ian is available? It's like, when you really like someone and over-analyze everything about yourself because you just want to impress them and seem perfect (and can come across as super awkward as a result). In any case, we see that in her desire for validation in how she dresses, and how she insists on butting in when Ian goes to talk to someone other then her - she wants to feel like the most important thing in the room for Ian. Or the most important thing for Ian, period.
I feel like her hooking up with Jeremy when Ian turns her invitation down at the start is to try to make up for how that rejection damaged her self-esteem, and because in that moment Jeremy probably gave her what she needed - a feeling of being wanted and to feel sexy. We see exactly the same thing if Ian grinds on Emma at the club - Allison will instantly start grinding on Jeremy. My guess it that's her way of trying to feel, or prove, that she's not deeply affected by Ian's rejection in that moment - to try to smooth over the sting by distracting herself with another guy.
I'm also guessing that Jeremy zeroed in on her emotional vulnerability like a pro - he's probably got a nose for it being the player he is - and of course he wouldn't say anything to Ian about it even if he knew or suspected, so he can take full advantage of it.
I'm not sure how well I explained that, but hopefully you get the idea.
