So let’s say you have a game. It’s a full game, excluding the main scenes for the intro. the full game has textures and what-not. Is it better to add multiplayer AFTER or before you finish making the game? I’m thinking of using python or the add-on for the multiplayer part
Its best to work multiplayer into the base of the game, so I would say during the development of the game 
Neither. You don’t build one then the other, you do something called planning.
Sit down with a piece of paper, and plan how things are going to communicate. Normally the keyboard interacts directly with the game objects, but with multiplayer, does this change? Does the multiplayer take the commands to a server?
Or is control done locally, with just object positions transferred through a server?
Plan how disconnects, connects and reconnects are handled.
But you don’t build the multiplayer, then the game. You build both at the same time.
To be honest it’s a good sign of development if you can build both separately. Generally speaking one would advise to build multiplayer first because it sets you into a mind set when developing the game. In many ways, multiplayer is an extension of the engine, hence you need to learn how to use your engine before you can use it to create a game. Planning is an important first step, but design in practice will demonstrate in occasion a need to change design due to a limitation
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