Code:
on 500:TEXT:!ylist*:#: {
  var %a = 1
  if ($ulist(*,0) == 0) { msg $chan I have 0 results in my user list. }
  while $ulist(*,%a) {
    var %namestotal = $addtok(%namestotal,$v1,32)
    inc %a
  }
  msg $chan I have %namestotal in my user list.
}


That should work. $addtok adds names to the %namestotal list and prevents duplicates.

Optionalled, if you know you don't have duplicates, you can replace with:

var %namestotal = %namestotal + $v1