Create a playlist containing the tracks you tagged ‘group’ using a tool outside roon.
(you likely got one since you were albe to tag the files)
Import this m3u playlist into roon.
Open the playlist and to a LONG-click upon the first title.
Scroll down and SHIFT-click upon the last tite in the playlist.
use the … (3 dots) at the very top next to where it says Edit xxx tracks or similar)
and all the selection to a new roon tag you might want to name ‘group’.
you could then delete the playlist since you could get the tracks using the new roon tag now.