My random theory...
For whatever reason, Corey strikes me as a selfish DB, who's not all that smart, and who probably has knowledge or an inkling of what went on, but not enough to actually do anything about it.
JR's car was in the shop, but he has a friend there from out of town. I doubt that any friend, no matter how good, is going to help him remove a body.
MB by all appearances, is a nerd who probably hangs in the shadow of his more popular, more suave, more connected, and better looking friends.
So with those thoughts in mind, I think that Corey was probably pretty effed up too. He had pretty bad motives bringing Lauren back to his place - he was bringing her back to hook up. Once he gets back, he starts hurling and the party (for him) is over. He passes out and goes to bed.
MB is then alone with a hot girl, the likes of whom he'd never normally have a chance with. She's being cuddly and sweet in her drunkenness. Mike's "rescued her" from the handsy Corey. He takes advantage of that and takes some liberties with LS but then realizes she's passed out. OR Corey stumbles across them as he's heading to the bathroom, and being out of it, doesn't know what's going on except that Mike is with Lauren by himself, which is weird. Either way, it's time to get her out and now so there's no chance she'll remember what he tried or did. He brings her to JR's, saying that she got the drugs/booze there, so he's responsible now. He leaves.
JR starts to notice that Lauren's flitting in and out of consciousness. Maybe she's starting to show some signs that things are going south. JR's first thought - this is my



, I gave her this stuff. He slaps her across the face to get her attention. No response. In desperation, he grabs DB's keys (his car is in the shop and DB's been long passed out upstairs), and Lauren, intending to dump her off at the hospital (and drive away). Sometime into the brief drive, he realizes she's dead. The ER dump is no longer an option. What to do, what to do? He drives to some location (where??) - and hides the body. Comes back.
Everyone has something to hide. Corey's afraid to say anything - he's too dumb to cover up - and what he was trying to do can make him look bad, really bad. It could set him up as a potential rapist. Hence the amnesia story, which isn't that far from the truth with how wasted he was.
Mike doesn't want people to know what HE did. And in a moment of panic, he says something to more astute JR (When you saw Lauren did she mention me? did she say anything about me?), who calls him out and accuses him of rape or taking advantage of Lauren. MB stops talking because JR has caught him out.
JR has to have a legit excuse to get LS out. He says he had her do the "sobriety test" and that he watched her walk out. It makes him look like a good guy, and it effectively ends his responsibility.
He says bye to his friend, DB, with DB being none the wiser about what happened with his own vehicle. He hires a lawyer courtesy of mommy and daddy, and life goes on.
The three all know something bad about the other guys - but they all have something on each other. It gives them a reason to protect each other to keep things quiet.