I've done this a lot in the past editions; in our current game our DM has one of us running an NPC.
I've seen it work very well.
Two (possibly three) benefits:
- Your players get to try out another class by running the NPC, so make sure the NPC is a different class than the character
- Your workload goes down, and your players have to stay busy/engaged to keep up with both characters.
- (Maybe) They may start to play better tactically, with more options in the table.
Caveat
Now and again, since these are NPC's, you have to step in as DM and offer up "your NPC won't do that" when, for example, the player decides that's it's time for the NPC to make a suicide/kamikaze charge into certain death ... (unless you have already determined that the NPC has a death wish ...). You'll also need to fill in various background things that fit into the story that only the NPC's might know.
Since you only have two players, this also allows you to set up encounters closer to the "basic" game design threshold of 4 PC's for the XP budget in the DMG. In time, you can (if the NPC's survive) allow the players to each run 2 PC's. That's going to be something you and the players need to discuss and explore as an option if it arises. (Done that frequently in the past, it can work ... )
T.J.L.'s answer pretty much sums up the RAW on the question: NPCs, if they have participated in combat, will get an equal share of experience. That is the general rule. What this also means is, for non-combatant NPCs like Stool and Shuushar (both of whom I like to send to the back, where they won't take up 'camera' space), when they have absolutely no participation, get no experience.
On the NPCs leveling up, they do not have character levels, so they wouldn't. It is also ill-advised to give them or increase their potency with XP gain, since, especially early on, some NPCs simply outgun the typical 1st-level PCs (Jimjar, Eldeth and Derendil are particularly strong!). This would result in PCs lagging behind as the NPCs statblock 'levels up' at the same rate as the PCs level up. Remember, the PCs are the stars! And by 4th-5th level, they will start to feel a lot stronger than their NPC friends.
However, I have run Out of the Abyss without sharing experience with the NPCs, and it seems to work out fine. We're nearing the close of the first half of the adventure (Blingdenstone still unexplored), and the characters are at 5th-level.
Chapter 7 mentions that:
The characters should be 8th-level by the start of [chapter 8]. If they're not there yet, plan additional encounters so that the characters can earn more experience points during
their overland journey to Gauntlgrym
To me, and observing the pace at which they are leveling up with XP divided among NPCs, this note sounded very odd. I didn't want to throw-in unrelated "fillers" to bridge the level gap, dividing XP with all the NPC party members exacerbated this, so I forewent it in favor of a better paced story.
TL;DR Yes, NPCs get a share of XP as a general rule, but in this module's case, it is not needed.
Best Answer
I've been frustrated for months with the NPCs in Out of the Abyss
I've been DMing Out of the Abyss for my group for months now, and the sheer number of followers has frustrated me for a substantial portion of that time. I actually had the very same goals that you do: keep the NPCs viable but in the background. I didn't want them to disappear during combat, but I also didn't want them overshadowing the players.
I found it impossible to balance the game around the number of NPCs that the party could have at any given time.
I tried changing health points around as you suggested, and I tried grouping up NPCs into distinct units based on the mass combat rules of in the DMG. I tried just having them fade into the background, upgrading their equipment over time, and I even tried giving each NPC a full turn in combat and letting the PCs run the NPCs in battle. And every option had the following problems:
For all of these reasons, I switched to a support system instead of statting the NPCS and I suggest you do the same.
If you look on page 114 of OoTA there are certain benefits for completing tasks for the Svirfneblin. Some of those include benefits that the PCs can call on for free that represent Svirfneblin aid in battle without actually running any NPCs. I took this and built a system on based around it. Here is the gist of that system:
I found that this system was much better at addressing the number of NPCs in the party. It allows the players to spend more time doing cool stuff with their characters and it doesn't cause the massive combat bloat that running each individual NPC does. It also solves most of our issues. NPCs aren't as strong as the heroes, are purely supportive in nature, and stay in the background at all times to let the heroes shine. If you want, you could even give individual NPCs new benefits to show how they are developing with the players. For example, I myself gave an NPC a new benefit to add extra fire damage to a player's attack after the players gave that NPC a magic flaming sword.
Monsters in my system can't attack the NPCs though
Since each benefit comes with a risk, monsters don't directly attack NPCs in this system. Instead, when something bad happens to the NPC after providing their benefit, I simply narrate how their actions got them injured by one of the monsters, without actually using any of that monster's action economy. Still, I was perfectly willing to make that concession for all of the other benefits that this system provided.
If you want to see exactly the system I use, you can view it in this google doc