I honestly don't understand any of that. If we're using a I-IV-V progression, how do the ii and vi come into play? Also, how would someone know what progression to play just by sitting in. I guess if I knew a song in G and knew it followed a certain progression I could transpose to C, for example, but how else would you know what to play?
The chord progression being different in two cases. Any chord progression you get off the Internet is some one's interpretation. To get the artist's chord progression you have to purchase the sheet music, and good luck with that as the publisher may have it wrong.
How did the ii and vi get in there. The ii and vi are flavor chords and are thrown in "willie nelley", in Country I doubt they really have anything to do with harmonization (are needed to harmonize the melody line, just something the artist does).
How do you know what progression to play in a jamming session? You don't, but, if you grab a I IV V and listen and watch what the other guys are doing you will be close enough. Jamming or sitting in with no sheet music you are expected to do your best and flow with what is going on. Don't make waves and do not step on toes. Your job is to keep the beat and call attention to the chord changes. That is done with roots and fives and chromatic runs to the next chord.
Yes I know, if you are asking questions like this you have not played enough to flow with what is going on. So how do you know what to play? Well, Country is going to be the ole I IV V7 with some ii and vi thrown in. So you miss the ii and vi, the sky will not fall. Keep trying and it will come, to jam along you have to have played a bunch of Country.
Your other post on Dixie and the instrumental break. The bass in a Country band will not be getting lead breaks, the lead electric or pedal steel take the lead breaks, so don't worry about that instrumental break. We play the harmony for the melody, i.e. we play chord tones. While the lead electric is taking that lead break, you just continue to lay down the same bass line you were using, in this case it will probably be the ole R-5-R-5. It's now his solo, so back off and give him room, lower your volume, but, keep playing the beat as he will need that to augment his efforts. If you just want to know - a safe way to take a lead break is to follow the chord progressions and play the chord's pentatonic. No not in pentatonic scale order, the tune enters into the picture. Using the notes of the chord's pentatonic scale make a melodic rendition of the tune. Good luck, takes years.
Little something on chromatic runs in Country. Song in C F & G (I IV V) You are on C and F is coming up. OK leave early and grab the C# and D, now go up a string and get the E and land on the F for the chord change. Now you are on F and G is coming up. Leave early, back up and grab the E then F, F# and land on G for the chord change. The chromatic walk is target the next root note and miss it by three frets, then walk to it and be on it for the chord change. The Country walk is a 3 note chromatic walk. You can walk one note, two notes or three notes, which ever one works best in this song.
I IV V and chromatic walks to the next chord is what we do in Country. Most anything else will get fish eyes from the guys. Takes a lifetime to perfect that, so get started.
Hang in.