How can I find a player not using localplayer?

Read the title above

There are several methods.

On a server script, I use the Players.PlayerAdded event to register the player when they join the game.

game:GetService("Players").PlayerAdded:Connect(function(addedPlayer)
	print(addedPlayer.Name.." has joined the game!")
end)

You can use Players:GetPlayerFromCharacter() as well, which gets the player from their character model in Workspace.

Door.Touched:Connect(function(bodyPart)
	local Character = bodyPart.Parent
	local Player = game:GetService("Players"):GetPlayerFromCharacter(Character)
end)

Hope this helps!