How can I group labels in a Prometheus query?

Yes, you can you use label replace to group all the misc together:

sum by (new_group) (
  label_replace(
    label_replace(my_metric, "new_group", "$1", "group", ".+"),
    "new_group", "misc", "group", "misc group.+"
  )
)

The inner label_replace copies all values from group into new_group, the outer overwrites those which match “misc group.+” with “misc”, and we then sum by the “new_group” label. The reason for using a new label is the series would no longer be unique if we just overwrote the “group” label, and the sum wouldn’t work.