Many , many years ago my older brother landed the lead guitar slot in a new band. They asked if he knew anyone who played bass, he recommended me.
I'd owned a bass for a couple of months but didn't have much idea of what to do with it.
The first rehearsal I sat at the back , kept my head down and did my best to keep up with root notes of the chords. My brother and the rest of the band were seriously good players, I thought that I wouldn't be asked back.
The following week , my brother was fired and I was asked to stay on.
Just do what you can, in my experience being thrown in at the deep end was a good thing, don't panic.