If we come as a group, is the $8 fee for the tour guide per person, which would increase the rate from $12 to $20 per person?

You do not really need a tour guide at the Museum of the Bible

There is no rate per person, the museum is free. If you are paying, then you are on a scam website. You can make a donation if you want. You can pay for a guide if you want, but you probably won't need one. They also have a tablet+headphones combo for free for a self-guided tour.

You don't need a tour guide.

We were over 40 and we didn't get a tour guide and It was still great!

We were given a tablet with earbuds as a self-guided tour.

The museum as a free entrance. They require documentation, so basically is whatever you want to pay. So it only be the tour guide fee as an extra

Yes it would be $20 dollars. Also there are some side attractions that are $8 as well.