Summoned creatures die when reduced to 0 HP

The general rule for what happens when a creature runs out of hit points is:

A monster usually dies or is destroyed when it drops to 0 hit points.

For PCs (and important NPCs), they have a more specific rule that allows for a "second chance" of sorts:

When you drop to 0 hit points, you either die outright or fall unconscious [and then roll death saving throws each turn]

Now in 5e, specific rules always trump general rules; However, there is no rule specific to summoned creatures that overrules the general rules above.

As such, a summoned creature will die if reduced to 0 hit points, unless that specific summon has a different rule governing it's reduction to 0 hit points. Demons are one example of having their own rule (as found in the Monster Manual):

Outside the Abyss, death is a minor nuisance that no demon fears... When a lucky hero manages to drop a demon in combat, the fiend dissolves into foul ichor. It then instantly reforms in the Abyss, its mind and essence intact...

If PCs were summoned by some kind of magic, they would be at the same risk as if they were teleported there normally.

How to summon them

Now apart from just teleporting the players to the destination directly (either by having the players cast a teleportation spell or by having a courier teleport to the players and then teleport away), there are very few official options that work.

Gate

Gate allows you to create a portal that pulls a target through...

When you cast this spell, you can speak the name of a specific creature (a pseudonym, title, or nickname doesn't work). If that creature is on a plane other than the one you are on, the portal opens in the named creature's immediate vicinity and draws the creature through it to the nearest unoccupied space on your side of the portal.

... but this only works if the players are on a different plane to the destination, and would require casting gate for each creature that needs to be moved. Not to mention it is a 9th level spell.

True Polymorph and Drawmij's Instant Summons

The combination of these two spells could, in theory, conjure the party to any place; However, this option necessitates the party having been objects the entire time between when the summoner(s) cast the spells on the party and when the party is summoned.

Essentially, the summoner(s) cast true polymorph on the party turning them into objects of some description (emphasis mine):

Choose one creature or nonmagical object that you can see within range. You transform the creature into a different creature, the creature into a nonmagical object, or the object into a creature

While the creature is an object, the summoner(s) cast drawmij's instant summons, and then at some later time break the sapphires conjuring the party (which are still objects) to the summoners:

At any time thereafter, you can use your action to speak the item's name and crush the sapphire. The item instantly appears in your hand regardless of physical or planar distances, and the spell ends.

Then the summoner(s) simply dispel true polymorph.

Wish

Since both of the options above involve multiple castings of 9th level spells, wish seems to be on a lower power level, as it may only require one casting. Wish allows for essentially anything to occur at the behest of the GM:

You might be able to achieve something beyond the scope of the above examples. State your wish to the GM as precisely as possible. The GM has great latitude in ruling what occurs in such an instance; the greater the wish, the greater the likelihood that something goes wrong.

Instantly transporting a group of people to your location is certainly within the realm of these beyond-the-scope wishes, so it could be a viable method; However, using wish in this way does have a cost:

The stress of casting this spell to produce any effect other than duplicating another spell weakens you. After enduring that stress, each time you cast a spell until you finish a long rest, you take 1d10 necrotic damage per level of that spell. This damage can't be reduced or prevented in any way. In addition, your Strength drops to 3, if it isn't 3 or lower already, for 2d4 days. For each of those days that you spend resting and doing nothing more than light activity, your remaining recovery time decreases by 2 days. Finally, there is a 33 percent chance that you are unable to cast wish ever again if you suffer this stress.

You're the GM

While the official options available to you each have their own limitations and all necessitate very high level magic, you are the GM and can choose to use different options at your discretion.

If it benefits the story, just do it. You could explain it as a ritual that requires 6 10th level casters (who could each just cast teleportation circle to bring the party anyway) and it summons willing characters across any distance. Really, any option is open to you as a GM; you aren't limited to the same options that players have.

Summoned creatures are minions.

And therefore, they must be commanded to get actions.

The chain of looking this up for yourself:

1. Summon Fey is a Conjuration spell that says you summon a creature
2. The Conjuration School of magic notes that "creatures summoned by conjuration spells have the summoned trait."
3. Summoned states that "It has the minion trait."

Note that for summoned minions, the Sustain a Spell action to keep the creature summoned doubles as the action to command it (as noted in the Minion trait)

And from this, Minions only get actions when you spend actions to command them, unless they have an ability which allows them to take some actions otherwise (like Rangers' Mature animal companions, who may be able to spend an action against the Hunted Prey):

Your minion acts on your turn in combat, once per turn, when you spend an action to issue it commands.

Which states that it only acts when you spend the action.