Context
My 5e homebrew world is set in the same cosmos as Faerun, but on a different world, so this is a Forgotten Realms question.
I want to introduce a mercenary guild that issues magical items to clients that can be used to call upon the guild's mercenaries with the press of a button. Teleportation would be an obvious choice, but I also wonder about "summoning". It is my understanding that in D&D 3.5, a summoned creature reduced to 0hp did not die, but rather was merely sent back to its native plane (PHB 3.5, pp 173 as cited here). This seems like an excellent mechanic for hiring out mercenaries – after all, there much less risk of them dying on the job.
From a gameplay point of view, I thought it would be fun to create this guild as a source of side income and one offs for my players. Sometimes I feel like they want a change of scenery, but are reluctant to derail their current adventure progress, so perhaps this would be a fun way for them to be propelled into a quick, action-packed encounter in an exotic location, and then sent back home when they're done. Yes, I could just use teleportation magic, but I'm interested in this mechanic because it potentially reduces risk in what is supposed to be a downtime activity for my players.
Question
Can PCs be summoned by another creature and if so, what are the drawbacks of this?
What I'm having trouble with is figuring out the limits and drawbacks of "summoning", and if these types of magics can be used on PCs at all. I can find no confirmation that summoned creatures in 5e don't die when reduced to 0hp. If this is permitted, what stops a group of PCs from fighting every BBEG as summoned creatures?
Additional Information
Note that the two "Summon Demon" spells are not the only summoning spells: all of the "conjuring" spells use the word "summon" in their description text.
Best Answer
Summoned creatures die when reduced to 0 HP
The general rule for what happens when a creature runs out of hit points is:
For PCs (and important NPCs), they have a more specific rule that allows for a "second chance" of sorts:
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):
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...
... 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):
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:
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:
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:
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.