The flask has no effect on most PCs while on the Material Plane
When you target a creature with the flask:
If the target is native to a plane of existence other than the one you’re on, the target must succeed on a DC 17 Wisdom saving throw or be trapped in the flask.
This effect only happens for creatures not native to the current plane. No effect is specified for a target that is native to the plane of existence, which means that if you are on the Material Plane and target a creature native to the Material Plane, the flask has no effect. The vast majority of playable races are native to the Material Plane, so they generally cannot be trapped by the flask under normal circumstances. However, there are exceptions, such as Eladrin (mechanically a sub-race of elves), who are “native to the Feywild”. An eladrin on the Material Plane can be affected by the flask.
However, if the party were to make an excursion to another plane, such as the Feywild, the PCs would indeed be vulnerable to the flask’s effect while they are not on the Material Plane. Conversely, an Eladrin PC would be immune to the flask while on the Feywild, since it is their native plane.
It may even be beneficial for a PC to use the flask on another PC under some rare circumstances, since the requirement to obey another PC’s orders might help override an enemy charm or mind control effect. This would obviously require quite a lot of trust, both in and out of character.