Originally Posted by: kellydROCKo

Lets say the band says the song is in the key of "C"

How do you know what to play to join in?[/quote][p]You have to know:

1. The chord progression: which chords, in which order, duration of each chord.

2. The form of the song: verse, chorus, etc. & the chord progression for each.

This of course presupposes you have the physical skills to play the chords in the specific rhythms being used.

[quote=kellydROCKo]

But have no clue as to how to use them to just get up and play with others without memorizing the song first.

Learning a bunch of songs is how most people actually learn to jam or improvise. Once you can play a variety of songs, then you have a library of chord progressions & types of riffs at your disposal that you can mix & match as you desire.

So, learn & memorize a bunch of songs. Then mix & match the various parts. Now you are jamming!

Gary does some really interesting lessons on how to figure out songs or what notes to play when you are jamming.

https://www.guitartricks.com/lesson.php?input=30479

https://www.guitartricks.com/lesson.php?input=29594

https://www.guitartricks.com/lesson.php?input=29495

Hope this helps!