I would think that it works like stroke play or match play, where the green speed defaults to the highest ranked player in the game.
From the FAQ, at the bottom of the page......
When playing with other players, the green speed
will default to the green speed for the highest-tiered player in the
group. For example, if two Amateurs and one Pro player are playing in a
group, the green speed will be "Fast" based on the Pro player default
speed.
Happy hitting