You need to be on Monica's corruption path, complete their joint scenes at the beach, and just keep progressing through the story.
You can't switch to the corruption path if you've already chosen the love path. Other girls won't be added, since the "girl status" only shows the main story girls — more characters will appear in Season 2. There will be 3 main endings in the next update, one of which will be the canon ending that smoothly leads into Season 2.