Here is part of an old one of mine that does what you want
This one will show you the list of channels you are in in the top list box and will show you the nicks in the channels when you double click on the channel name

Menu Menubar,Nicklist,Channel {
  .Venom Profile Viewer: dialog $iif($dialog(profileview.system),-v,-md profileview.system) profileview.system
}
Dialog profileview.system {
  Title "Profile viewer"
  Size -1 -1 207 80
  Option dbu
  list 1, 02 02 100 40, hsbar vsbar
  button "view profile ",2, 02 62 37 12 , flat
  list 3, 02 33 100 40, hsbar vsba3
  edit "type nick to check here", 4, 40 62 62 12, autohs
  list 5, 105 02 100 75, hsbar vsbar

}
on *:dialog:profileview.system:init:*: {
    chanzz
}
on *:dialog:profileview.system:dclick:1: {
  nickzz
}

alias  nickzz {
  var -s %curr 1, %nick $nick($did(profileview.system,1).seltext,0)
  did -r profileview.system 3
  while (%curr <= %nick) { 
    did -az profileview.system 3 $nick($did(profileview.system,1).seltext,%curr)
    inc %curr
  }
}
alias chanzz {
  var %chans $chan(0), %curr 1
  did -r profileview.System 1
  while (%curr <= %chans) { 
    did -az profileview.System 1 $chan(%curr)
    inc %curr
  }
}

This one will just automatically load in the nicknames of the Active channel when you open the dialog.

Menu Menubar,Nicklist,Channel {
  .Venom Test Dialog: dialog $iif($dialog(testdialog.system),-v,-md testdialog.system) testdialog.system
}
Dialog testdialog.system {
  Title "Test Dialog"
  Size -1 -1 207 80
  Option dbu
  list 1, 02 02 100 80, hsbar vsbar

}
on *:dialog:testdialog.system:init:*: {
   nickzz
}

 alias  nickzz {
  var -s %curr 1, %nick $nick($active,0)
  did -r testdialog.system 1
  while (%curr <= %nick) { 
    did -az testdialog.system 1 $nick($active,%curr)
    inc %curr
  }
}