User blog comment:B214/Volume 22 spoilers/@comment-50.139.232.230-20161022060406/@comment-9595671-20161022142753

Yeah, all the members who joined after Issei are the ones she traded to him, the ones who bonded with him more than with her because they hadn't gained the same history she had with Akeno, Kiba, Gasper and Koneko.

While Rossweisse didn't ask to join Issei (that I know of atleast) I'm sure Volume 17 helped her become more confident about herself, especially after hearing some more of her past and what Issei told her. For all we know she asked Rias to do this off-screen.