When upcast, the etherealness spell lets you target multiple creatures. The spell’s description includes the following termination clause (emphasis mine):
You step into the border regions of the Ethereal Plane, in the area where it overlaps with your current plane. You remain in the Border Ethereal for the duration or until you use your action to dismiss the spell. During this time, you can move in any direction. If you move up or down, every foot of movement costs an extra foot. You can see and hear the plane you originated from, but everything there looks gray, and you can’t see anything more than 60 feet away.
Prematurely ending the spell seems straightforward for the caster, the caster simply spends their Action to dismiss the spell. However, this portion of the spell was written for the spell under its normal casting criteria (i.e. not upcast) which has a typical target of Self.
Compare this to the language used in the astral projection spell, which by default targets other creatures:
The spell ends for you and your companions when you use your action to dismiss it. When the spell ends, the affected creature returns to its physical body, and it awakens.
If you are returned to your body prematurely, your companions remain in their astral forms and must find their own way back to their bodies, usually by dropping to 0 hit points.
So in the event that etherealness is upcast and affecting multiple other creatures, does the caster need to spend their Action to end the spell for everyone? Or does each character need to spend their Action to dismiss the spell on themselves? Can the other characters even use their Actions to end the spell or could they be trapped on the ethereal plane until the spell runs its course?