We've had full-party-member NPCs since forever and rarely if ever has it been a problem. They come about in one or more of several ways:
--- Someone's hench graduates to full-member status (henches are uncommon but not unheard of)
--- The party realize they're lacking some skills and recruit NPCs to fill those holes (this is the most common party-NPC generator)
--- The adventure gives the party an NPC as part of its story e.g. a rescuee with adventuring skills, and that NPC fits in well enough to stick around after that adventure's done (also fairly common)
--- The NPC is a long-term DM plot device, or a double agent, or a spy, or will eventually reveal itself at some key moment to be part of the opposition (rare, but it's been done; most often the PCs cop to the NPC's real motives early enough to avoid any real headaches)
--- the PCs meet and merge with another adventuring group, each seeking strength in numbers at the time (rare, but it's happened a few times)
--- the party take in tolerable members of a rival adventuring group they just defeated (surprisingly common)
The players do the dice-rolling for the party NPCs, track their hit points etc., and usually determine what they do in combat (though the DM can always veto); the DM does their role-playing, treasury claiming (just like any other party member), and so forth.
I usually try to keep party NPCs at or near the level of the PCs; sometimes below, rarely if ever above unless it's something dropped in my lap by a published module. The one subtle difference is that often party NPCs will come and go from the party more often than PCs will, which tends to slow down their advancement. I also rarely if ever get attached to these NPCs; they have a normal sense of self-preservation but if they die, they die, and I don't lose sleep over it (edit to add - though sometimes the players/PCs will go to great lengths to revive a fallen party-NPC, treating it just as they would a PC party member; which makes sense given that in the fiction there's no 'PC' or 'NPC' labels stuck to the characters' foreheads).
That said, party NPCs have personality and will interact with the PCs based on that: rivalries with PCs, romances with PCs, friendships with PCs, etc. are all in play and - I hope - add to the entertainment.