I can only confirm this for shaman class, specifically the buff "Foreboding"
In a group, shaman casts buff on tank. This buff takes 3 concentration slots. Group then evac's. Shaman's concentration slots are now open but buff is still on tank. Shaman now casts buff on another member. New member has buff, tank STILL has buff, and shaman is using the normal 3 concentration slots.
Can't confirm if this works with any other spell/class combo, but it works for this.
Hope to see premium access coming my way soon
trogdormajesty