After Mitch and Mickey are introduced during the concert, there is an overhead distant shot of the stage. Mitch can be seen nodding to the guitarist to his left, and the two begin strumming - but no music is heard. Mickey also begins playing and her lips don't move, but she is heard thanking the audience. It then cuts to a two-shot, and nobody is playing while Mickey continues talking to the audience.
In the final Reunion Scene, you can obviously tell that none of the actors are actually playing guitar. Most of them just move their fingers like strumming, and you can clearly see no vibration from the strings or contact with the fingers and strings.
When Mitch and Mickey are introducing their song at Town Hall, Mitch's hand is making a "C" chord. In the next shot with just Mickey's head and Mitch's guitar in the background, it has no hand on it. When the shot returns to Mitch and Mickey together, he is still making a "C" chord.