If a unit (Shen) is played to a location during an attack (after attackers are declared), does it get affected by Ahri's ability that affects attackers?

Ruling: Yes, units are considered attackers even if they arrive at the location after attackers are initially declared. All attack triggers related to them fire after the cleanup that assigns them as such.

Nuances:

• The same principle applies to defenders
• In the specific scenario asked, Shen couldn't actually be played because the player doesn't have control of the battlefield during an attack