If you're not using formats, I'd use print instead of printf. If you've got quotation marks in your quoted string, try using qq. If "grp" really is a filehandle you opened with open or something, I'd recommend putting it in all caps, or better yet use a lexical filehandle.

# open my $grp, '>', $out or die "Can't write '$out': $!";
print $grp qq{<test:tester username ="$id" group-name="$grp1"/>\n};
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