After a bit of trial and error, I have managed to get Ian to sleep with both Emma and Holly (slutty) at the beach. A few things I discovered while trying to accomplish this; First, you can sleep with Emma (Jeremy too) and still pair her up with Perry by the end of chapter 12. The two key choices to look out for are: when she ambushes you in the toilet at Fortress during Lena's concert, reject her advances. If Ian is dating Holly or Lena, he will explain he does not feel comfortable doing it while they(Holly and Lena) are close by. She will take this as you being serious with them and stop pursuing Ian (you can finger her ass at the beach later in chapter 12 but she just takes this as Ian just teasing her
). From there on help Perry at least pay for half of the BBQ costs and side with Perry when Emma brings Jenthe to the beach house. However, if you reject her at the bar and you are not dating Lena or Holly, she still maintains her interest in Ian. If you still want to set her up with Perry, you have to pick to share rooms with Wade and turn her down when she comes to have sex with Ian the next morning. She will then stop pursuing Ian.
Now to sleep with Holly and Emma at the beach, you have to either have broken up with Lena or be not currently dating her atm. This will allow you to choose sleeping arrangements (this is the key to make this work). Now when you get to the beach, choose to share rooms with Wade instead of Emma. Regardless of this Emma will still come to fuck Ian in the morning while Wade is passed out on the couch. Finger her ass on the beach. You can even tell her you "feel the same way" when Ian asks her if she's got her eye on anyone. Come night time though you don't get the second sex scene with Emma since Ian is sharing a room with Wade (As a result of this the group is still oblivious to the hi jinks of Ian and Emma). On this playthrough after Lena broke up with Ian because he slept with Holly, I still made her give Ian a blowjob at Blazers and they ended up heavily flirting with each other during the beach event. She even let Ian put sunscreen on her boobs. Come night time Emma will reject Perry but because of the sleeping arrangements she won't come to visit Ian. Holly however has no such reservations and picks this moment to make her move
. Ian and Holly do the deed, and Holly plays coy the next day. Perry is being a dick to Emma cause she didn't bang him but he doesn't get mad at Ian cause he is not privy to what's going on between Ian and Emma.