Does Exalted still apply if additional creatures are later included?
That is correct. Exalted triggers, and the creature that's attacking alone gets +1/+1 until end of turn. It doesn't matter whether that creature continues attacking alone for the rest of the turn (of course, it can't once the combat phase is over).

There's a Gatherer ruling on Angelic Benediction to this effect:


  10/1/2008: Some effects put creatures onto the battlefield attacking. Since those creatures were never declared as attackers, they’re ignored by exalted abilities. They won’t cause exalted abilities to trigger. If any exalted abilities have already triggered (because exactly one creature was declared as an attacker), those abilities will resolve as normal even though there may now be multiple attackers.