If you only need/want to display all User assignments (ignore role-only and team assignments) of the parent task or issue in an assignment report:
1. add below text mode to a new column in your assignment report:
displayname=All Assignments
listdelimiter=<div>
listmethod=nested(task.assignments).lists
textmode=true
type=iterate
valueexpression=CONCAT(IF(ISBLANK({assignedTo}.{ID}),"",{assignedTo}.{name}))
valueformat=HTML
2. add below text mode to a separate new column just to the right of the above column:
listdelimiter=<div>
listmethod=nested(opTask.assignments).lists
textmode=true
type=iterate
valueexpression=CONCAT(IF(ISBLANK({assignedTo}.{ID}),"",{assignedTo}.{name}))
valueformat=HTML
3. Save your report.
4. Open/edit report
5. Add the below line of text to the (left-side) column from above:
sharecol=true
6. Save, you're all set.
Doing the above should result in the below text mode in a single column, but the column numbers (4 and 5 below) are likely to be different based on how many columns are in your report:
column.4.displayname=All Assignments
column.4.listdelimiter=<div>
column.4.listmethod=nested(task.assignments).lists
column.4.sharecol=true
column.4.textmode=true
column.4.type=iterate
column.4.valueexpression=CONCAT(IF(ISBLANK({assignedTo}.{ID}),"",{assignedTo}.{name}))
column.4.valueformat=HTML
column.5.displayname=All Assignments
column.5.listdelimiter=<div>
column.5.listmethod=nested(opTask.assignments).lists
column.5.textmode=true
column.5.type=iterate
column.5.valueexpression=CONCAT(IF(ISBLANK({assignedTo}.{ID}),"",{assignedTo}.{name}))
column.5.valueformat=HTML