Jamming - no sheet music. Some one will call out the key; "Next one is Kiss Ole Kate in G, ready 1 & 2 &..." Use your fingers and the alphabet to find the I - IV - V chords in that key. Yes, it's not rocket science.
The I IV V chords will be a friend. Why? To get harmonization the melody line and the chord line should share like notes when they do you have harmonization and both lines sound good with each other. The I IV V chords will have in their make up every note in the I tonic's scale. So..... those three chords will sooner or later harmonize the melody. Yep, one like note (roots) gets harmonization, two like notes (R-5) is a little better. Three like notes are not really necessary, the root and the five have given you what you need more is just gravy. So, ....... while the melody is using primarily notes from the I chord we use the I chord for our bass line. When the melody moves on to notes not found in the I chord we will need to change to a chord that has some of the new melody notes in it. Good news - you have a 50% chance of hitting that new chord and if you missed it you have a 100% chance that the other chord is going to work. Wish someone would have told me that years ago it explains how music thinks.
Assume the verse will start with the I chord, move to the IV chord near the end of the first line of the verse. Continue on into the second line and then near the end of the second line the progression will move to the V chord and quickly resolve back to the I tonic chord to end the second line. The next two lines of the verse will repeat this same progression. Is it like this on every song? No, but, you will be close and in jamming it's like horse shoes and grenades - close is good enough. If you are jamming, with no sheet music, assume the above and trust your ear.
Assume what chords will be used then watch the rhythm guitar's hands and change chords when he does. After awhile you will recognize his fingering and know what specific chord he is going to. More good news - in a year or so your ear will tell you which of those three chords to use.
Remember all you need are three chords and just root notes of those three chords will let you jamm with anybody.
Go have fun.
Key of C = C F G
Key of G = G C D
Key of D = D G A
Key of A = A D E
For the other keys - use your fingers (thumb, ring and pinky) and the alphabet.