Can I Turn My Single Player Game into a Massive Multiplayer Game?

I'm currently working on a game project that I'm finding would be quite enjoyable if massive amounts of players could login and all play at once. This is merely a hobby project that I do for fun and to learn new things. At this point I'm curious as to the order I should develop this game in. If someone intends for potentially hundreds or even thousands of players to play their game, should a server be worked out first? Is it possible for me to simply keep developing the game without networking capabilities and then down the road add the feature for others to connect and play at once? I've read mixed advice online and admittedly I'm a little confused.