Plugins mostly have not changed from how they worked in Videojs 4.x. You still register and use them the same way. The only major change is that if you are instantiating a plugin inside the config of a player, this plugin will get run before the player is ready. This is so that if plugins are doing some UI work or adding themselves as children of the player they can do so early on. This means that plugins that require the player to be ready would need to handle that themselves or else not support instantiation via the player config. Ex:
videojs('my-video', {
plugins: {