who | cut -d ' ' -f 1 | sort -u
who: will show who is logged on
cut -d ‘ ‘ -f 1: will remove all sections from each line except for column 1. It will use the space character as the delimiter for the columns
sort -u: it will sort the usernames and remove duplicate lines. So if a user is logged in multiple times you will get that username only once.
In case you want to filter out root user from this list you can do it as follows:
who | cut -d ' ' -f 1 | sort -u | grep -v 'root'