Use the DISTINCT Keyword.
$query = "SELECT DISTINCT grp FROM Users WHERE grpnum = 1 ORDER by 'grp'";
the output will give you 1 distinct result for every different group name associated with the group number.
You could also use the GROUP BY Clause that will pretty much do the same thing
$query = "SELECT grp FROM Users WHERE grpnum = 1 GROUP BY grp ORDER by 'grp'";
If you are sure that only one group name is associated with one group nuber, youdo net need the ODER BY clause