Quote Originally Posted by zlefin View Post
Do you have those NPCs thoroughly statted up?

What is the reason you don't want the teamup to occur?

My first inclination is to simply the party, OOC, reasons why you don't want the teamup to occur.

A hypothetical example:
"I know it makes sense to teamup from a verisimilitude perspective; but DM'ing is a lot of work, a teamup requires me to alter encounter balance significantly, it also means I have to run those NPCs all the time in addition to everything else I'm doing. So let's just not do it except for very special occasions, ok?"
I think this is the best option, assuming you don’t actively want to try to make it work. The players in a D&D game can sort of do anything that makes sense from an in universe perspective, but not really. The game does have some basic assumptions and it will fight you if you stray too far from them.

If you do want to give it a try, I’d suggest relegating the NPC group to roles where they won’t need to actually be run as if they were PCs. Run a normal encounter where the NPCs are fighting other enemies in the background. You can even come up with a simplified combat mechanic to see how they do, and maybe some of them will die or get wounded, or switch sides or something. It’s not impossible and there’s potential for fun there, but I personally would steer clear of actually adding them to the party or treating them as DMPCs. Even one DMPC is one too many imo.