A bass guitar has four strings, not three, and it is a different instrument, as a French horn is different from a tuba. Both are played by vibrating your lips and blowing into a mouthpiece, but they are considered vastly different instruments. A six string guitar is plucked or strummed, whereas with a bass, your fingers "walk" the strings. There is also a special relationship between the drums and the bass, that doesn't exist with the rythmn or lead guitars. Also, the bass player is always the coolest member of the band. LOL!! ;-D
I haven't figured out the bands thing yet, either. I am awaiting the delivery of the Prima Guide, though, and I hope it helps.
EDIT: I've figured it out! Talk to your selected soon-to-be Band Member. Under the "Friendly" conversation button, you will find the "Form a Band" option. Enjoy!