Here's the code.. it is tested, but as it uses the /list command, you will get the list window.. I was not able to figure out a way around that.
Code:
menu * {
  Number of Users : .llist $?="Enter channel name (wildcards will work)"
}
alias -l llist {
  .enable #llist
  .list $1
}
#llist off
raw 322:*:{
  .echo -a $2 has $3 users
}
raw 323:*:{
  .disable #llist
  haltdef
}
#llist end