Spent way too long figuring this out myself when I jumped into 'The Lying Game' series. The core sequence is pretty straightforward: start with 'The Lying Game', then 'Never Have I Ever', 'Two Truths and a Lie', and finish with 'Hide and Seek'. That's the main story arc following Sutton Mercer and Emma. But then there's a prequel novella, 'The First Lie', which actually slots in before the first book chronologically. I read it after finishing the series and kinda wish I'd done it first—it adds a lot of texture to Sutton's character, but honestly, you won't be lost if you skip it.
After 'Hide and Seek', Sara Shepard wrote 'Cross Our Hearts', which is a sequel set a year later. Some fans argue the series ended perfectly at four books and that the fifth feels like a bonus epilogue, but I was just happy to spend more time in that world. So, publication order works perfectly, but if you're a completionist who loves chronological depth, kick off with 'The First Lie' novella.