How are the guards recruited?

There's an interesting theory regarding the first game Gi-hun is offered to play at the train station, called Djaki. Note that there is a blue card and a red card. Gi-hun chooses the blue card and he gets selected as a player (cyan outfits). The theory is that, had he chosen the red card, he would have been recruited as a guard (magenta outfits). This would explain why the guards are also being regulated in some manner (numbered rooms, time schedules, no talking policy, no revelation of identity).

Source

There is no certain information revealed, so unless there will be a second season explaining it, we can never know for certain.

However, there are many strong hints.

One such very strong hint explains how they are loyal and why they don't mind killing unarmed innocents: both the host (on his deathbed) and the front man (when executing the doctor for cheating) explain an ideology they strongly believe in: they believe they're giving a fair second chance to people whose lives were already beyond saving in the outside world. This ideology can be like a central doctrine of a cult. (Yes, the games were created by the founders purely for their own entertainment, but creating this ideology is a good excuse for them to suppress any traces of conscience they might have had, or their employees might develop)

Another key point is that the contestants are given a fair choice. Everyone is there on their own accord, they aren't forcing anyone to compete, and they can have a fair vote at any time to stop the games. Of course, we can call them hypocrites if we want (circumstances of the player's lives force them into continuing), but that doesn't matter. All that matters is that the organizers strongly believe that they're not responsible for the player's deaths, because they are there on their own free choice. This means that any of the staff who might have conscience problems, can use the above justification to not feel guilty.

We should not forget the Milgram experiment, and how prone guards in POW and forced labor camps were to mistreat inmates. Especially if they can find an ideological justification to see them as subhuman. And here the players are people who are already "beyond saving" in the outside world. They will likely get killed by loan sharks, or commit suicide pretty soon. They can be considered as being already dead. Remember, some guards referred to the contestants as "zombies". So if they're already dead, why should the guards feel guilty about shooting them? Especially if the players are there on their own free choice?

So, where can they get new recruits?

• we see (if I remember correctly) 3 unmasked lower-rank guards, and all of them are very young. They can be recruited/kidnapped as children, probably as orphans, and raised/brainwashed that way. The organizers are strong believers of the "free choice" doctrine, even if that free choice is only "free" from a certain viewpoint, but if they really recruit orphans who have nowhere else to go, they are "providing them an opportunity" just like they do with the players.
• higher ranking guards might be strong believers of the ideology presented above. It's like high-ranking people in a cult, some might be there for strongly believing the cult's teaching, others just for enjoying being in power over people they can feel superior towards. The organizers of the games are some of the richest and most influential people on the planet, I won't be surprised if they were also owned stakes in some cults.
• former winners are not enough to make up the full roster, many are unlikely to want to join, and it's unlikely that they are forced to do so, given the central doctrine of "free choice". But some of the higher ranking staff can be former winners, just as the Front Man is a former winner.
• a very likely pool of candidates is made up of those who voted to leave and didn't return. In the current year, there were 14 people who decided not to return, and the Front Man is seen giving the order that they should be kept under surveillance. Remember, the candidates are all in a desperate situation, with debts they are incapable of ever repaying. It is very likely that when new positions open up, they are approached and given the opportunity to join the staff. They aren't forced, but they don't really have any choice besides getting killed by the loan sharks they are indebted to or dying of starvation. They are desperate, and probably were on the verge of choosing to return but chickened out at the last moment because they were too afraid of dying suddenly in the game. But they are desperate, remember that 93% of the players decided to return to the games on their own free will even after they learned that they get killed if they don't win. The other 7% is equally desperate, so if given a choice to be member of the staff, where they have a much higher chance of survival, they'll be eager to accept.