My preference is to induct the original group even if a solo member had a slightly better career. It really hurts no one to do it this way and the main solo act will get most of the spotlight anyway. The solo star typically wants all the attention and likely makes demands behind the scenes to get inducted solo (Beyonce vs Destiny's Child). Beyonce record sales are higher, but real physical sales are actually very close (after 2015, you can totally discount Billboard numbers with their arbitary stream equivalent formulations).
Having a successful solo career isn't as hard as being an independent act from the beginning. It is more of a continuation and their fans typically liked the original group and the performers usually perform the group hits as well.
With the backlog, you cannot waste multiple spots. Multi-inductees should be extremely rare.
Rock Hall inducted Jefferson Airplane. They are not worthy alone (2 moderate hits and poor sales). If you combine Starship, Jefferson Starship, and Hot Tuna offshoots, then the induction makes more sense and gives them a much better case.
Seeing Yukio's list, it will be a bit worrying for the future of the Rock Hall, unless the Rock Hall just nominates and inducts 2–3 or more lesser classic rock acts every year to "fill the spots," or they reduce the number of inductees. Otherwise, the ballot and inductee quality will eventually drop a lot after ten years. The majority of the very obvious 80s and 90s alternative, rap, and even a bit of R&B/pop picks would be finished in ten years...
Then finally, we will reach the point where, for example, The Cranberries are top rock candidate on the ballot against Avril Lavigne and Creed... Or they will finally have to nominate better-acclaimed hipster acts (PJ Harvey, Pavement, The Flaming Lips, Modest Mouse, The National, Yeah Yeah Yeahs, etc.).
Lax43: Thank you for nominating X Japan - that was going to be my nominee, but with them nominated and Selena (who would have been my other nominee) already on the ballot, I'll go third option: