Need help

I need to write a script to check to see if a co worker is logged into server when I get to work. I am new to Unix I know that I need to use the Who and grip command to check and an if statement. Can anyone help me?

Output needs to be

check and see if Bill is in
NO

check and see if Bill is in
No

check and see if Bill is in
Yes

cat atwork.sh

#!/usr/bin/bash
COWORKER=${1}

echo "Check and see if ${COWORKER} is in"
who | grep -i ${COWORKER} > /dev/null
if [ ${?} -eq 0 ]
then
  echo "Yes"
else
  echo "No"
fi

Execute:
# atwork.sh bill

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