Here are a few tips I found that might be interesting for anyone who likes including some NPCs as more or less permanent members to their party in ToEE, and are looking for a few new angles for party planning:

Burne
Normally, if Burne is in your party and you loot the Golden Orb from Falrinth's 2nd chest, as soon as you exit from level 3 of the Temple a dialogue will come up saying Burne returns to his tower in Hommlet and is no longer hirable as an NPC.

This however, is only the case if an active PC holds the Orb in their inventory. You can instead loot the Orb to an NPC other than Burne to keep, and Burne will no longer leave the party (in my game, I gave the Orb to Otis for keeping). That NPC can even use the orb in combat without activating Burne's quit dialogue. Apparently the presence of the Orb just bothers Burne if the party leader wants to use it.

Other than the Orb dialogue activating, Burne can freely leave and rejoin to the party as often as desired, even staying on into Verbobonc if you are playing Co8 mod.

Otis
Normally Otis is hard-coded to leave the party each time you return to Nulb, and he refuses to freely leave the party unless you are in Nulb. The first time he leaves, you can have him rejoin for free, but the second time he leaves, he leaves for good. Of course you could always just never go back to Nulb and he would never leave your party. But sometimes you might want to go back if you have unfinished business in Nulb, but otherwise hesitate because you don't want to lose Otis's services.

Well, if you attack a "good" aligned NPC anywhere in the Temple (let's say by casting a Charm spell on the concubine Paida for example, or knocking her unconcious) Otis will turn hostile and leave the party. If you then successfully cast a charm spell on hostile Otis, he rejoins as a follower (AI controlled) rather than an active NPC. As soon as the charm wears off (hang out in Wonilon's Hideout as long as needed, or march back-and-forth to Hommlet a few times), Otis will go back to Nulb on his own AND FORGET HE WAS EVER HIRED A SECOND TIME. So you can basically return to Nulb and rehire him (getting the same dialogue as before saying he will only go with you one more time). Btw, Paida also returns to the location where you found her in the Temple once her charm spell wears off, and forgets she ever even met you. So no harm done.

You can use the above described technique to "fire/rehire" Otis from/to the party as often as you like.

Domination
You can basically add ANY NPC in the game to your party by using the 5th level arcane spell Dominate Person, as long a you have one of the 3 available NPC slots free. It works on both hirable NPCs as well as those that are normally non-hirable or even hostile. So, for example, you could add Serena or Raina in Nulb without paying their exorbitant hiring fees. Or you could add any of the random non-hirable NPCs standing around in town or any prisoners you find in the Temple. Or you could add the (normally hostile) clerics Kelno, Belsoring, Alrrem or Romag. Or maybe you prefer having the "Curry Leaf" fight for you instead of for the Temple?

The spell should last well over 100000 game-rounds, or roughly 1 week of game time. Have a new Domination spell ready if you want to "re-add" the same character once the spell wears off. Otherwise, they return to where you originally found them (with any equipment and extra exp they happen to get).

Hope these tips might give some players new ideas for having some extra NPC fun while playing through their adventure of ToEE!