I just bought a 7 person hottub and don't know if I have to drive a copper stake for the ground.

You should read article 680 of the NEC, it deals with the very strict requirements of the installation of pools and Hot Tubs. Or check with your local Electrical Inspector for requirements. This is something you should really want to get right. A Ground Rod (stake) will not be allowed, you must have a solid connection to the electrical supply ground wire.