How do I make team-only seats like owner-only doors in tycoons?

It's not too complicated, what you're looking for is probably Occupant which is a property of seat which returns the humanoid that is occupying the seat, which is kind of self-explanatory.

You can have the seat connected to the .Changed event so when someone sits on it, it will fire and then you can use Occupant and find the player's name like so:

( this is example code and I wrote this up quickly it may not work)

local seat = script.Parent

seat.Changed:Connect(function()
	local occupant = seat.Occupant
	local occupantName = occupant.Parent.Name
	local player = game:GetService('Players'):FindFirstChild(occupantName)
	if player then
		if player.Team == BrickColor.new('Really red') then
			-- kill the player here
		end
	end
end)