How can I make this work? it doesn't work after running is it because the player hasn't loaded? I look on roblox documentation but they don't explain much as how to they get their id? By character or humanoid something...
for i, v in pairs(game.Players:GetPlayers()) do v:GetRankInGroup(12345) > 1 then print("Not a member") end
You're using > which is "Greater Than" so, you're printing that they're not a member even if they are. Also, use 0 instead of 1, as it will still print not a member if they're the first rank in the group. Hopefully this helps, if you don't understand anything, I'd be happy to try to explain better!