Jericho-79,
In the situations you describe, you and your lead player are the songwriters and everyone else is contributing to the arrangement of the piece. You are free to credit yourself and the guitar player as the writers or to credit all of the band members if you feel their contribution warrants it. It's your (and the guitar player's) call.
As an example, listen to "All Along The Watchtower" by Bob Dylan and then listen to Hendrix's version: they're VERY different, but Hendrix and the boys don't get writing credit for coming up with some cool riffs, beats, fills and solos. Dylan wrote the song, Hendrix and the boys just rearranged it.
As to Bernie's comment on chord progressions, there aren't very many chord progressions, so can't really be copyrighted. (Don't get me wrong - if you come up with something truly unique, it could copyrighted, but unless you invent your own tuning system and chords, you really aren't coming up with something that hasn't already been done in classical, jazz, etc.). Otherwise all those jazz songs would violating existing copyrights because they're all either ii-V-I progressions or the rhythm changes progression, all the blues (and half the rock) songs would be in violation because they're all I-IV-V progressions, and so on. Besides, those progressions date back to classical and jazz compositions so old they're in the public domain already, anyway.
