The multiplayer mechanics of Elden Ring make it quite easy to play through nearly the entire game alongside a friend or two, but even setting a password won't stop other players from invading. In fact, those who have been playing a lot of co-op may feel as though they are becoming the target of invasions an inordinate amount. Setting a multiplayer password restricts the pool of available summon signs, but doesn't affect how Elden Ring's PvP invasions work. As long as players are participating in co-op, they are liable to be invaded.
Elden Ring's brand of multiplayer harkens back to 2009's Demon's Souls, but is far more forgiving than early iterations. Being in Human Form in Demon's Souls - or having restored Humanity in Dark Souls - would open the player up to invasions. This mechanic introduces a trade-off where summoning is possible, especially in Demon's Souls where characters in Soul Form have lower maximum hit points. In Elden Ring, this trade-off has been transferred entirely to co-op. Playing co-op in any form will open up the game to invaders, a consequence for the massive boon having a second player can be.
This rule is universal, regardless of whether or not co-op has been initiated through the use of a password. Elden Ring co-op helps with leveling up and fighting particularly tough bosses, but the entire affair comes with an inherent risk of being invaded. This mechanic also means the host will usually be at an advantage with two players against the one. For those frustrated with invasions, there's unfortunately no way to circumvent this, since it's a purposefully designed element of Elden Ring's multiplayer.
Elden Ring Multiplayer Passwords Won't Stop Invasions
Even using a multiplayer password doesn't stop the game from allowing invaders to enter the game. Selecting a password simply limits which summon signs can be seen and who can see a placed summon sign. The multiplayer password doesn't affect invasion mechanics in any way, but those who keep getting killed by skilled red phantoms can take an extra precaution. The Twin Maiden Husks in Roundtable Hold sell an item called the White Cipher Ring for just 1,000 Runes. The White Cipher Ring is another way Elden Ring's co-op summoning works, and will invite another allied phantom when getting invaded.
If already playing in co-op (where most invasions happen without using the Taunter's Tongue), this shifts the odds even further in the hosts favor, making it three versus one. This isn't a perfect way to get more help against invaders, though, since it relies on others using the Blue Cipher Ring to answer the call at the same time. PvP can be a difficult part of FromSoftware games, especially since those who do it regularly tend to make character builds specifically for invasions and duels. There's no easy way to avoid Elden Ring invasions while playing co-op, even with a password, so players will just have to stay on their toes.