I really want to know what their first meeting was. We know they invited themselves to his house, but how exactly did they all end up with each other. Were they friends at first? Was it immediate? I wonder if we'll ever know.
Going by chapter 1, plus the original H-doujin that this is a continuation of, they never had anything to do with one another until Hamasaki, the blonde, overhead Imaizumi sticking up for her when his friends were talking about how she had to be a slut.
That led to her talking to him and the three of them inviting themselves to his house and it apparently wasn't long before Hamasaki jumped him, since the H-doujin's first chapter mentions it's only been 3 months since he started living on his own.
So, TL;DR: it probably wasn't
immediate immediate, as in same night, and they weren't exactly friends at first, but they also weren't strangers and it wasn't long. Maybe a week or two, since this series picks up after a month after the H-doujin's first chapter.