Tente isso,
users=( User1 User2 User3 User4 )
for i in "${users[@]}"
do
grep -qw $i file && echo "$i is in the file" || echo "$i is not in the file"
done
De man
:
-q, --quiet, --silent
Quiet; do not write anything to standard output. Exit immediately with zero status if any match is found, even if an error was detected.