It’s only a rough estimate, but I compared a sample of group workouts to similar rides I had taken solo with similar metrics (same course, similar duration, average power output, total calories, elevation gain). I also compared my posted speed during a group workout to my posted speed under the same conditions riding solo. I did this for multiple scenarios. I tried to keep everything as comparable as possible so as to isolate the miles only.
There was a little variation but it was reasonably consistent: on average, it looked like the group workout miles showing were about 2/3 to 70% of what they would have been otherwise. I imagine from person to person and from group to group this may vary, this is just where my numbers seemed to land.
So on my spreadsheet log, I show posted miles, but added an additional column to show the equivalent in ‘normalized’ miles. Does that make sense?