I turned multi select off and now I get a report with "multi select" the name on every group in my report. However, the Assigned to is not multi select and there is only one name in the assigned to.

Did more digging and found that ALL sheets the report is looking through must have multi checkbox off.

I've had similar results as described above when trying to Group using the Assigned To field in a report. However, I also discovered that you can create a sheet with multiple Assign To option turned OFF. Then create a report using that sheet and establish a group on the Assign To field. Then go back to the sheet and turn ON the option for multiple select (in the column properties) and add multiple contacts into the rows as needed. The report will continue to work but will auto adjust to allow for the multiple contacts selection. It will give you the pop-up warning, but the report keeps working.