Welcome to Chordie Juan.
http://www.ezfolk.com/uke/Tutorials/1fo … chart.html
Use this chart and look at the three chords in gray. Learn any of these three and you can just improvise when the song they're playing is in this key. Ask them what key they are playing and you'll know what three chords goes along with the song. Many songs only use these three.
key of A: A D E
key of C: C F G
key of D: D G A
key of G: G C D
These chords follow the usual progression of 1 4 5 where 1 is the 1st key in the scale, and 4 or 5 is the 4th and 5th note in the scale.
So for Key of A, you may not can play Bm or C#m but you can hang with them when they are hitting the A, D and E chords.
As you progress, you'll find the Em, Am, and eventually the Bm and F#m can be learned. Hope you'll find yourself playing along mostly on some of their songs and in your practice time on your own you'll find many easy songs that only use the 1,4,5 chords anyway.
Tell us how it's going as you progress...and good luck!
Steve