When I use the ability on the Anduril Boon card from the Road Darkens, does that count as my only attack against that enemy for the turn? Or can I attack it with other characters later? Also, can other characters join in the attack with the hero wielding anduril?

Same as Quick Strike:

No.

No.

You can attack the enemy with other characters when it's the "player attacks" step of the combat phase; however, other characters cannot join in the special Anduril attack.