While grouped, the bard speed meoldy Tynen's Melody of Acceleration appears to not apply the movement speed to the group memebers but it does seem to deduct the movement speed when the song ends. So if you play the song and end the song a bunch of times in a row your group members go into the negative movement speed percentages.
The bard itself gathers up movement speed. The bard has become a movement speed vampire.
You can clear this by relogging.