The Bass plays the 'Root Note' of the Chord being played. That's the most basic way a novice will start off. Tempo, 1,~ 1,2. Sounds cool actually.
Enough guy's, go crank something up to 11 and unload, that's the reward of all the hard work !
Here's how the guy on ZZ Top plays most of his songs and sings while doing so.
Tempo, 1(hold),and 1 and, 2 and
OK, if the band is playing an E chord, his notes would be
Tempo, E(hold), D,E,D,E,D
Back and forth all night long with some minor variations.
I could be in ZZ Top. 
I'm sure I said 'Novice' somewhere in this very messy thread in reference to 1, ~,1,2.
Kick's ass; I'd say Yankee !
As long as the note is plucked to send a lovely fat bubble of Bass in unison with the Band, it's all good stuff.
joechimp, just wanted to add, you make me laugh regularly, thankyou.
