From now until March 31, 2020, if you have a half year or year ticket (Halbjahres/ Jahreskarte) you can take a friend for free in zone 101 after 6.30 pm Monday through Saturday and all day Sunday & holidays. (This also includes night buses). However this does not include reduced year or half year tickets, for example seniors or students.
„Bus Bahn Bim for two – Er/Sie fährt mit mir!“
And if you want to know more about how to use public transportation in Graz (for example what is zone 101) you can read my post here.