is this Player:IsNotInGroup() a form?

Hello, I have a question is, player:IsNotInGroup() a form?

Yes. But it's worded differently. If you want to check if a player is not in a group You can use Player:IsInGroup(GroupId). as an alternative. It returns a boolvalue of either true or false.

local player = game.Players.LocalPlayer --You can use game.Players.PlayerAdded as well.
local groupId = 0 --Change the 0 to your groupid.
if not player:IsInGroup(groupId) then --If you put a not, it will check for a false value, rather than a true. 
	--run code
end 

So in conclusion, no, it doesn't exist. Use this alternative.