My party has been fighting stuff that can travel to the ethereal plane and a few of us have been using blink, but I'd like for a way for some party members to be able to come back if they get stuck there. We have been using blink with people in a bag of holding to blink into the ethereal plane and drop the bag as a reaction. This campaign that my DM has created takes place on a plane separate from the material plane and does not allow spells that involve traveling to other planes that aren't ethereal so we cannot use Banishment, Plane Shift, Gate, and the like. So I'm wondering is there anything I can summon that has Ethereal Stride or a similar ability that can port us from the ethereal plane?
Call your friend the nightmare
You've identified the (to my knowledge) only method of bringing someone else from† the Ethereal plane. And that's the Ethereal Stride trait that's unique to Nightmares. Nightmares are fiends, however they are neither devils nor demons, so can't be summoned by infernal calling nor summon greater demons, leaving only planar ally (PHB p. 265).
That still leaves a few hoops to jump through. It's a 6th level cleric spell, and we're here presuming that in your campaign it summons the creature from your normal plane. On that plane you'll also need to know a demon prince (or similar) who can send you the nightmare. Then you'll need to sort of payment, the nature of which we'll leave to your DM after they've read the spell and monster descriptions. That might turn out to be the consequences for plane-hopping in a bag of holding going wrong. Emergency tickets home get expensive.
If you have the levels for it, you could of course true polymorph a creature or party member into a nightmare.
† There's a near miss with an upcast etherealness (PHB p. 238) spell which will let others move to the ethereal plane, however it doesn't work while on the plane and you can't target creatures across the planar boundary. Upcast etherealness is available to Ki-rin which are perhaps a nicer thing to get from planar ally. Also, similarly horse themed.