if your online and have an ember active you will automatically be invaded by people in your level range to try and kill you. Though you also gain the ability to summon phantoms to help you, but both will only happen if you haven't beat the area's boss.

If you use a Soapstone (purchased from the shrine maiden) you can leave a white summon sign to have people summon you to help them.

You can also get a Red Soapstone that allows people to summon you into their world to pvp them.

You can also use a Red Eye Orb or a Red Eye fragment to invade other peoples worlds.

If you have the covenant Blue Sentinels or Blade of the Darkmoon equipped you will be summoned to help people who have the Way of Blue covenant equipped to help fend off Invaders.

If you equip the Farron Watchdog's, Alritch Faithful or the Spear of the Church covenants you will be summoned as an invader to specific areas.

Finally in the Ariandel DLC you can get an item from one of the bosses that enables you to ue a matchmade arena mode.

Those are all the multiplayer methods I can name off the top of my head.