yes that The guitarist just plays first position cowboy chords and they take up a lot of the sonic room on their own.!
Cowboy chords normally will follow the I-IV-V chord progression with a vi or ii thrown in. So you miss the vi or the ii chord change - catch it in the next verse, i.e. don't sweat it. Would be nice if you could change with the I-IV-V.
Dirt simple rock or country will normally start with the I chord then near the end of the first verse bring in the IV. It then may bring back the I. If so you are back at the beginning so next chord is going to be the IV. Notice something up to now we have used the I and the IV. Go look at some Blues chord progressions you will find a lot of I's and IV's and just a couple of V's. So you miss a V, get it on the next verse. I's and IV's will keep you in the mix.
Every note of a scale will sound good with any other note of that same scale. Every chord made from those same scale notes is going to sound OK with all the other chord found in that key. So you land on a wrong chord, the World will not stop turning. Smile and keep going. Now if you have someone in the band with perfect pitch - nope - he will beg you to hit the right chord change. I've only played with two perfect pitch people so I doubt you having this problem. Jamming with out sheet music - - get as close as you can and if they still ask you back you are doing fine.
Back in the day I would stand so I could see the rhythm guitar's fretting hand and when he changed chords, I followed him to that chord. Course it does help if you can recognize his fretting patterns. If you can not ask him to show you what the fretting pattern for the basic 7 chords look like. Basic chords, i.e. C, G, D, A, E, B & F. Now to the next question........
..... Now about playing more than just the root. We see all the bass gods (notice small g) throwing in notes all over the fretboard and we say to ourselves; "I want to be just like that when I grow up." Being like that is not really necessary. A good steady beat will get you asked back. My suggestion; get the roots coming with the beat - kinda important - with the beat - stand where you can see the drummer's kick drum pedal; the rhythm of his kick drum is the rhythm you play your bass line to.
Need more than just roots add the 5. Still need more bring in the 8 which is just the root in the next octave. Roots, fives and octaves will play a lot of bass. When you get into Jazz you can start bringing in the correct 3's and 7's. For right now roots, fives and eights will handle most anything you will come up on. When you start playing some jazz add the correct 3 and 7. That kinda puts everything into perspective for me.
Nothing wrong with roots. It's how you play those roots, i.e. don't forget about the rhythm. The song will tell you if something more is needed. Yes, you'll hear. As others have said practice using the other chord tones at home so when you hear that more could be use - use more.
Have fun. That is why what we do is called playing.