Orzhov Advokist's ability doesn't target the creatures that receive +1/+1 counters. Their controllers choose them as the ability resolves. Players can't take actions between making their choices and the +1/+1 counters being placed.

If a player chooses to accept Orzhov Advokist's gift, that player can't attack you or a planeswalker you control with any creatures during their next turn, even creatures that weren't on the battlefield as Orzhov Advokist's ability resolved.

Orzhov Advokist's effect keeps creatures from attacking you even if Orzhov Advokist leaves the battlefield, if the creature that received counters is no longer on the battlefield, or if an effect has moved those counters off of the creature that received them and put them onto another creature.

While resolving Orzhov Advokist's ability, you choose a creature you control or choose not to put any counters on a creature, then each other player in turn order does the same. Each player will know the choices made by players who chose before them. Then simultaneously each player who chose a creature puts two +1/+1 counters on the creature they chose.