If I'm understanding your question correctly, you want to type in 'a' at the command prompt and get output of 'who' command.

Easiest way I know to do this is to add an alias statement to /home/[user]/.bashrc.

alias a=who

Here is partial excerpt of alias section of my .bashrc file:
alias nano='nano -w'
alias ls='ls -ahF --color=always'
alias cp='cp -i'
alias mv='mv -i'
alias rm='rm -I'

Also, 'man' command is your friend.
Try man alias
or man -k alias

hth
harv