To join a game, you must firstly have premium Minecraft (the paid version).
Moving on, there is a hub for 3 servers - au, eu and us. There are also as and ca servers. You should play which one best suits you. When you press multiplayer, you can add servers to your list - I would personally add the hub server of your region (if your region has a hub) >>> these servers are: au.mcgamer.net, eu.mcgamer.net and us.mcgamer.net.
If you join there, simply make your way to the Survival Games section and click on one of those signs to join a server. If the server is full you won't be able to join. You must join a server in the 'lobby' mode to participate; this is easiest when a game is about to end. To vote for a map, simply type /vote and your map choices will come up through 1 to 5. They will have numbers next to them. Let's say Breeze Island was number 3; you would write /vote 3 in this situation. Whichever map has the most votes will win >>> buying donator will allow you to join lobbies that are full, carry more votes and receive a pretty in-game rank.
You can also add in servers for your own region (personal ones). This will be specific MCSG servers - au1.mcsg.in is an example. You can join the server you want to directly through there.