JoinParty in scripts

I would like to create a spell that would add the target to the player's party--basically, a spell version of CTRL-Q. The idea is to create a charm spell that would let you bring the charmed creature to any area and trade items with them. This way, you could charm a spider from the Cloakwood and use it to poison Davaeorn, or charm Koshi and steal his katana, or charm a Shadow Thief and have him use the Staff of Striking to backstab Aran Linvail. It would be open to lots of abuses (Clay Golems would be invincible in certain fights), but I could find ways to balance it out.

Problem is, I don't know how. I think the only way to bring people into the party without using CTRL-Q is to use the JoinParty command via scripting, but I know little about scripts and do not know how to implement this using a spell.

Would it be possible to create a spell that could force the target to join the party? And would it be possible to make them exit the party after a certain amount of time?