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.
If you control more than one creature that can't attack alone, they can attack together, even if no other creatures attack.
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.
If a creature that can't attack alone also must attack if able, its controller must attack with it and another creature if able.
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.
