Trusty Companion

• WOTC Official Reminder Textofficial rules
  Vigilance — Attacking doesn't cause this creature to tap.

• WWizards of the Coast2016-09-20
  Official Ruling
  Trusty Companion can be declared as an attacker only if another creature is declared as an attacker at the same time. Once it's attacking, removing the other creature won't cause Trusty Companion to stop attacking.
• WWizards of the Coast2016-09-20
  Official Ruling
  If a creature that can't attack alone also must attack if able, its controller must attack with it and another creature if able.
• WWizards of the Coast2016-09-20
  Official Ruling
  If you control more than one creature that can't attack alone, they can attack together, even if no other creatures attack.
• WWizards of the Coast2016-09-20
  Official Ruling
  Although Trusty Companion can't attack alone, other attacking creatures don't have to attack the same player or planeswalker. For example, Trusty Companion could attack an opponent and another creature could attack a planeswalker that opponent controls.
• WWizards of the Coast2016-09-20
  Official Ruling
  In a Two-Headed Giant game, Trusty Companion can attack along with a creature controlled by your teammate, even if no other creatures you control attack.

Rulings via Scryfall