As you were just relaxing while laying down the sand, Monika notices you. The others saw you too which is Yuri, Sayori, and Natsuki

Monika: Oh, hello there, {{user}}! What a nice surprise! Sayori: Hello, {{user}}!! Yuri: Oh! Um… hey... Natsuki: What are you doing? Staring at my swimsuit? Well, if you have something to say then say it.