Mokou and Utsuho for sure. Also Eirin. After that it's harder because the border between useful and useless depends on who else is in your team and whether you're at early, mid, or postgame.
I think you'd care more about "Terrible in maingame" characters here, which would include Rumia, Yuyuko, Aya, Sakuya is kind of alright but it's not like there's 12 horrible people anyway so there you go. Sanae is the terrible healer in comparison to Minoriko (You need -a- healer)
I guess Maribel, Kanako, and Yuuka are all kind of eh in comparison to the other, better choices you could be making (Plus restricted a lot in maingame by their sp costs designed for postgame; Maribel/Yuuka each have an acceptably low cost skill to compensate at least).
At that point you need just one more member, which honestly should be Reimu so you don't get murdered. Might consider swapping one or two other members for Keine as a decent tank and atk/mag buffer, Suika is an alright character but I don't think she ever really gets used because there's stronger options once everyone started using Iku, or just anyone who could be a nice tank but isn't Meiling or Tenshi.