I got only three of the groups! It didn't matter in the end because I worked it out. If you've got all the letters, anagram them... This will give you a name - and then the highlighting will be straightforward.
Thanks Neveracrossword. It took me a while.
But you can slice the full name into 4 slices and find each slice in the grid.
Each slice can be a part of a group. But who cares.
I am assuming that the number is 1, 7.
However it's third letter (out of 7) is part of the name. So I get 23 cells highlighted.