Hi @PrivateAnalytic ,
Please refer to my pbix file to see if it helps you.
Create a measure.
Measure = CALCULATE(COUNT('Table'[PersonID]),FILTER(ALL('Table'),'Table'[PersonID]=SELECTEDVALUE('Table'[PersonID])))
If I hvae misunderstood your meaning, please provide pbix file (or some sample data ) without privacy information and desired output with more details.
Community Support Team _ Polly
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.