If controls are hard break people in slowly. Not every game uses every button, and not every button is used at the same time. Start with something simple, and most people become familiar within a few hours. Don't start straight online into a serious firefight, cos getting your ass kicked over and over is discouraging to noobs. I remember the first time I played my first FPS, I was so confused as to what did what, and got killed so much it put me off the genre.
The key is to break them in via the main singleplayer, or preferably the co-op multiplayer. Help them make progress, not by doing all the hard stuff for them, but by guiding them into what does what, or how to defeat certain enemies. Give them pointers, but don't patronise, just let them go at there own pace. Once they're used to how things work, then take them into online, and encourage them. Don't join a free for all, play team games, until they think they're ready for all vs all. Once they want to do it, and are enjoying it, then they'll be more inclined to keep going, even if they lose by a ridiculous margin. Better that they're enjoying it, and wanting it, than you're forcing them.