I used to be a tutor for something similar to this, run by the Music Service in Swindon under the name 'Rock School'. The skill level of the adult students varied widely, but they were all there to enjoy playing together, and especially for the buzz of a live performance.
Come the concert, you will find the audience very supportive of you all. Nobody will mind (if they even notice) the odd mess-up, even if it seems cataclysmic to you. To quote Brian Eno: In music, you can crash and burn, and walk away.