This is a copy and paste from the Smartsheet Basics community. I am having the same issue and would like for this functionality to be added to Smartsheet.
Grouping a Multi-contact Column on Report
I want to group a contact column with multiple contacts on a report to have a visibility of the tasks assigned to the the contacts.
When I look for the column under the group dropdown, the column des not appear. However, when I uncheck the allow multiple contact per cell button, the column appears.
So, I want to find out if a contact column (with allow multiple contact) can be grouped.