Multiplayer Question for Somewhat of a Beginner

I'm kind of new to Dark Souls, though I have beat the game and also beaten Bloodborne and am having an awesome time with it. However, I am quite confused as to the multiplayer. I am pledged to the The Watchdogs of Farron covenant, and when I am summoned to Farron Keep to protect it or whatever I am confused as to what is going on. I thought it was as simple as kill the host of embers, but there's clearly more to it. Sometimes I am immune to the enemies, sometimes I'm not. Sometimes the other people there attack me and such, other times we're all attacking the host. I'm sorry for being a total noob or whatever but if somebody could just explain the fundamentals here that would be great. I'm finding myself unsure of who I am supposed to fight in these situations and getting killed as a result.