Pretty small deficit if your ear is good. When you say you "introduce a bass line", do you mean one that you created? If so, I'd suggest that the key doesn't really matter. You could try telling your keyboardist (respectfully) to simply listen to the line and play what fits. It's certainly a useful skill to be able to name a key, but if you're composing original music with band mates, they'd be best served by just using their ears.
If you're playing covers, there's already some good advice here about finding the key. I'd just add that it's important to note that naming the key of a song isn't strictly science -- there's some flexibility, and unless you're dealing with written music, don't sweat it too much. If you can at least name the bass line's notes and take your best guess at which note represents the tonic, that'll get you pretty far.
. It wasn't clear to me because after it hovered around G for a bar or two, the notes went in a direction my mind couldn't classify. He said, "I don't know... it's just notes." I think I initially found his response frustrating, but it became a revelation as I realized that I shouldn't try to force every piece of music into a single key. One could undoubtedly study his piece and define keys for the various sections, but what he was really saying to me was, "Don't limit yourself to notes of a key (at least not on this piece). Just listen, absorb, then play."
