Column 'Comments.Text' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause
Your question isn't really clear, but I think you want something like this: 

select group_id, 
       item_id, 
       comment
from (
   select group_id,
          item_id, 
          comment, 
          row_number() over (partition by group_id order by item_id) as rn
   from the_unknown_table
) t
where rn = 1;


(you didn't state your DBMS, so this is ANSI SQL)

SQL Fiddle demo: http://sqlfiddle.com/#!12/a8471/1