$ cat userlist.sh

# Name of the server to be prepended to each line?
SERVER=R18-PRMY

# Nothing user-editable below
NOW=`date '+%Y%m%d'`
FILENAME=users_CMS-$SERVER-$NOW.txt

# Create or Empty the file
> $FILENAME

lastlog | awk -v server="$SERVER" '{print server " " $0 "\n"}' | tr -s '\n' > $FILENAME

Gives output like below. I don't see a flag in lastlog that turns off headers.

R18-PRMY Username         Port     From             Latest
R18-PRMY cms              pts/14   1.2.3.4          Mon Dec 18 16:59:22 -0500 2017