@sloperoller please don't tag solo members of a prsk group with their group tag. As far as I know, it's used for pictures of all 4 members of a group (as tagged by @Julius543 ), or for versions of vocaloids present in that specific group's sekai, like the Miku with white hair and heterochromia (as tagged by @luntoer ).
Don't really have any fast rules on it myself. Just noticed the tags a couple days ago and thought it a good way to distinguish the Mikus. Alternative would be to give them each their own character tag as is usually the case.
I use the Project Sekai band tags similar to BanG Dream band tags, at first I used it if there are all band members in the picture (Vocaloid characters don't have to be present) and now I think the band tags will be expanded to the band's costumes (this may be applied in other similar games).