So you skipped a game that has multiplayer, for whatever reason, and now you have the opportunity to play it. I understand the feeling. Jumping into a multiplayer scene late is an awful risk, but there are ways to overcome.
1. Is it worth the time?
If it is dead, it is dead. Multiplayer scenes die all the time, which is a terrible thing because people really enjoy certain games competitive modes and can not play them. Also, some games get plagued with hackers which could put you in a bad spot. So figure out if it is worth spending time learning it, or grind for your few achievements and be done. Also, some publishers shut down their servers and some do not so that would be great information to have before you really sink your teeth in.
2. Find people who are better than you
When I first started playing competitive games it had been months and years since these games came out. I eventually met people online who kicked me in the dirt and for me allowing that threw me some pointers. For example, in Halo 3 this guy invited me to a private lobby and he destroyed me. He paid attention to how I was shooting and told me to that hit location and learning the weapon matters a lot. Someone might think that is simple, but it helped me become so much better.
3. Make friends
If the servers are never removed you and the people you meet will always have that one game you all can play. This is a cool way to meet new people and join a community. Also, friends always make games more fun. You could also convince people you know to buy the game as well and play with you.
4. Go to the Boards
Communities are often still active for awhile with a few people still wanting to play despite the dying population. Go to these communities on Reddit or Google Plus and find some people. If you are achievement hunting it works the same way, ask someone to play. People are generally very kind about it and are always ready to get gaming. This is also a good determination of whether you should invest time in a game or not. If the community is actively talking or is saying they are moving on, you should too.
5. Do not buy the DLC at first.
When playing games like Halo 3 and Modern Warfare there were always people playing, but I never bought the DLC. Do I wish I had? For Halo, yes and Modern Warfare, no. Halo fans were more dedicated to the series and they have systems in place that penalize people for cheating and leaving games. I could come back and my experience would still be great. I would also have people to play with. For Modern Warfare, hackers invaded that game and there was no moderation. The game had no penalization for cheaters and Call of Duty fans eventually move on after the hackers move in. Another big consideration with this is time. How much time do you intend to play and how much time do think the game will have until the population leaves.
If hoped this helps in anyway and keeps you playing the games you like. If you thought this was a waste of time comment so I don’t ever have to do this again. Have fun out there everyone.