Replying to Group portrait indoors, looking for ideas and advice

I just got hired to shoot a brass bands (around 20-30 members) group and portrait photos for a price high enough to make me uncomfortable. The shots will be taken on a large stage (already decided where by band) which already has lighting systems installed. There is no audience but i only have around 2-3h in that room. I have two months to prepare. Sounds like something i should not be complaining about i know. Let me explain...

My worries:
1. Shooting indoors
2. keeping a large group of people entertained "posing" for a long time.

Why is that worrying me?:
I do not own any lights/flashes/backdrops (Camera experience and glass wise i am well equipped), my client is aware of this and my little experience shooting indoor portaits. Shooting using just the stage lights is agreed upon with the client.

I love capturing moments as a bystander, unnoticed. This will be the exact opposite. It all feels like a huge leap into a field of photography (people skills?) that is very unknown to me.

How do i prepare to approach the big day comfortably?

TLDR: Metal tube musicians spook hobby photographer with big job