If you only want to sum comments per user you can use the following query:
SELECT count(*) FROM table group by user
If you want the concat comma-delimited string, I believe you would have to do it programatically. In other words...
(This is off the top of my head and may not be the easiest way)
You could query the table using:
SELECT DISTINCT user FROM table
loop through the resulting array and in each iteration use:
SELECT comments FROM table WHERE user = $row;
iterate through that array and write the results to a delimited string:
str .= "$subrow[j], ";
