I thought that would be pretty obvious, just keep choosing to spend time with her/putting points into choices raising her affection for the MC and the options to date her become available.
Maybe your PT was different than mine on the Hero/Swinging path, but I don't recall her being dominated much if at all. She's pretty much the one calling the shots for most of the 3somes and its about even in the swinging scene. Maybe that happens in the sharing scenes, but I successfully avoided that content.
On the sharing route there is one scene with either her boss or MC's boss where the guy is really dominant and she acts sub, really hot. Also I remember one scene in their home where they invite someone over and she is a bit submissive as well.
