[Tomisimo]

There are exceptions and nuances to the concept of the personal "a". For example, you often don't use it after the verb "tener"

Tengo tres hijos.

Also you can use the personal a with things that aren't people to accomplish literary personification:

Amo a mi coche. (No es cierto)

The opposite is true as well. You can refer to people in a non-specific way and not use the personal a. And this is where your example comes in.

Necesitan jugadores.
Necesitamos trabajadores.
Estoy buscando ayudantes.

For this last one, if I'm thinking of a specific person (or people), I'd use the "a", but if I'm thinking of a general concept of players or helpers, then I wouldn't use the "a".