I'm pretty sure that Lena and Ian are always going to be at the beach house. In the real world it would make sense for her to back out, considering she very likely hasn't developed strong bonds of friendship with the rest of the scooby gang, aside from possibly Emma, but since this this is a game and Eva would have to write an entirely different chapter with entirely different scenes, characters, and dialogue. This beach house trip is one of the big three events we're expecting to wrap up the story, so it would be weird to not include Lena at all in it.
I personally have always been under the assumption that Cindy will be going and this will be where her and Wade's relationship storyline will reach it's conclussion. Cindy and Wades "couple weeks apart" should be done, so if Cindy has been unfaithfull with either Ian or Axel and Lena and Ian haven't encouraged Wade to keep trying... it all just feels like a point of culmination. Cindy also creates a lot of drama with her jealousy / rivalry with Lena, the aforemention Wade story arc, and if her and Ian have been together.
Emma seems like the next threesome option depending on Lena and Ian's relationship. If they agreed to experiment and be open then this will be a scene. Otherwise she is likely there to be a partner to Lena, Perry, or Ian, because I don't think Holly has actually been invited. I, and I think some others, have always assumed that Holly would be there because she is the third MC at this point even if she isn't a PC. But I can't actually remember her getting a direct invite from Perry, so there might be an actual possibility of a chapter we don't see much of Holly. Depending on how she has been encouraged by Lena and Ivy, Holly could undergo a very fun off screen change, having spent a week with no buffer between her and Ivy.
If Perry is in a relationship with Cherry I could also see her getting an invite, and perhaps have a chance to reconcile with Lena. Additionally the added drama of having another Axel ex interacting with Cindy can make for a spicy narrative moment.
Excluding Holly for now, the only other two characters I can reasonably see making an appearance would be Jeremy and Allison. However, both of them I see maybe only showing up as like a three day weekend appearance, depending on relationship variables, meaning they might only be around for about half the chapter instead of being there the whole week.
So to wrap up my thoughts I think this is going to be a more intimate chapter with a smaller cast of characters and I strongly believe that this is a great event for the culmination of many of Cindy's storylines. We might get a phone or text conversation between Lena and her half of the cast, same for Ian if he's with Allison or Holly if they don't actually show up to the event.
