PichotM Has an amazing post, check it out!
function GetPlayers()
local players = {}
for i = 0, 256 do
if NetworkIsPlayerActive(i) then
players[#players + 1] = i
end
end
return players
end
Run that client side then create a Server event that parses that info, if you need it serverside.
Else just have this in your clientside file. 
Hope this helps!