The D&D team released sidekick rules that let you make NPC companions for your party using rules that are only a little simpler than regular player character creation rules. They have classes, hit points, levels, skill lists, equipment, spell lists, spell slots, and other features that player characters track on a minute-by-minute basis in D&D.
You might like resource management so much that these new rules are really appealing to you. Many people try more rules light games and give them up because they love more crunchy systems. If that's you, you'll love Unearthed Arcana: Sidekicks. But if D&D is already just crunchy enough for you, I have a better suggestion.
I made Companion rules that are faster, easier, and more fun. Click here for my version.
These rules are inspired by Dungeon World's henchmen rules, old school D&D henchmen rules, and my experience running Out of the Abyss in 5th edition D&D - the module where you start off running a game for a handful of PCs and ten NPC companions.
In my Out of the Abyss game, I reviewed other folks' custom companion rules, and ended up just asking my players to handle the stats for the ten NPCs. I caused murder, mayhem, mystery, and party splitting to get rid of as many of the companions as I could, in part because of the table time that it took whenever someone would say "I want to give this surplus magic armor to this NPC" or "I want to buy this NPC a better weapon." Or just the way it bogs down to have NPCs take their own turns in combat, make saving throws against effects that target everyone, and roll ability checks.
I made these companion rules treat companions more like magic items. They provide some bonuses that you get to use, but they don't take a turn in combat.
Take a look.
PS: I turned on document commenting, since this is just a draft I threw together in response to Unearthed Arcana: Sidekicks. If you have constructive ideas for improving my work, please drop a comment in the document.