The bus company which operates Malaga city to Granada and return is Alsa. As I am a new respondent I am unable to provide full links yet, but Alsa.es in your browser should do it.

You have to specify which journey. All seating is allocated when purchasing your ticket.

The bus station in Granada is out of the centre and requires, if making for the Alhambra, a taxi or two city bus journeys.

If you need more, ask.