When someone asks you to do something musically (sing or play in their choir/band/combo/whatever), and you doubt your ability to measure up, rather than beg off you should take it as a challenge and jump in.

The person doing the inviting may have a truer picture of your potential abilities than you do ~ you're much too conscious of what you can and can't do right now for you to be a good onjective judge of what you'll be able to do within a whole new context.

Try to take the attitude that an opporunity is being dropped into your lap. Even if you may not fully deserve the confidence being placed in your skills, somebody thinks you do deserve it, and you should be gracious enough to give it a shot.