Josie I assume was told by Lizzie that this was a thing and would have showed up with her twin, since I see Lizzie said she was coming. She's a little quieter than Lizzie, but will be friendly and try to get to know some people so please come be her friend. She'll also spend plenty of time with Lizzie (and Hope, if she comes). If she's feeling brave, she might even ask some girls and/or boys to dance.
I'm not sure if anyone would have told Rose (or Lissa) about the party since they showed up after it was announced on the network, but if she knows about it, she would come, assuming Lissa wants to. She'd stick close enough to Lissa to keep an eye on her, but will also mingle and try to meet some people. She's not shy at all and if she sees a guy she wants to dance with, she will definitely try and make that happen.