So we all love a little cheese right?  It's more satisfying to break the game than install a mod to do it.  A while back I saw a post specifically about how to make the drider join your party but I couldn't find it again and did some testing of the method in Act 3, with some choice NPCs, and wanted to share the results here.

You need to be at least level 7.  You need one party member that knows Polymorph and a hireling that knows Dominate Beast.  I prefer Kree.

You need to be able to single out the target NPC so that no buddies nearby will enter combat with you to defend them.  Go into turn based mode (for convenience), have your companion cast polymorph on the NPC, then have the hireling cast dominate beast.  Now the sheepy portrait will appear next to your hireling portrait like a summon.  You can see where this is going already.

The next steps have to be done quickly because you only have 5 turns of polymorph (might consider extended spell sorcery).  Leave turn based mode and go to camp.  Then immediately go back to turn based mode to give yourself time to fly over to Withers. Talk to Withers and dismiss the hireling.  Then leave turn based mode so the hireling has a chance to disappear, and rehire them in realtime mode.  Now the NPCs portrait will be next to your hireling's portrait.

From here, leave camp, save the game, and reload.  The NPC summon's portrait border should have gone from purple to brown.  They're now following your hireling around.  And if the portrait is no longer purple, you can dismiss the hireling and the NPC will magically pop over to Tav's portrait and now follow you around.

I first tried this on Helsik, because the Infectious Greed condition is OP.  Unfortunately I couldn't get Infectious Greed to last through a long rest.  So sad.  Maybe someone else will have better luck.  Side note,  first time Helsik enters battle, all her golden minions show up at Devil's Fee and continue to work themselves into every battle turn order, despite not being present so it's convenient to go back and slay them after they spawn.

I next tried Mitchia to pair with infectious greed because she has aura of murder and also Mitchia is bae.  I had to use Improvised weapon repeatedly to move her to the fast travel point outside the temple of bhaal because you can't return to camp in the temple. With Mitchia in particular, do not change zones to Rivington while she still has a purple border or things start to break.  Just get into combat or save and reload to get rid of the purple portrait border.  And also have some soap ready for when Mitchia joins the party.  Her bonus action hold person and high health pool are nice along with Unstoppable.

Other NPCs that I think would be cool to try next are Ptaris or Hope.  Maybe I'll just get the mod in the future since I want to do a cheesy run of act 3 with Infectious Greed condition the whole time.

X Should be a companion! Make any* NPC join your party