After trying Twitter, first @Avis and then @Avis_DE and some DMs back and forth, I was told to call Avis USA to get an answer to this question. Called and was told it will be $8 per day up to $50 max to add an additional driver for a rental in Germany.

So I guess bottom-line is that the Europe-based Avis Preferred Plus status gives you an additional driver for free as a benefit, but the US-based version does not.