I would never allow a completely untrained person that I don't know in the ranks without it having been agreed on before the event.
Our muskets should be respected as the firearms that they are. And that need training.
The same is required to be able to stand and move in formation and not mess it up and be a danger to people around your.
I find the idea completely horrible.
Also here in Denmark To have an public event of any sort we need a permit from the police. (this also include a concert, or market or similar, protest or whatever)
The moment there are any public present, we need a "3rd party" insurance. That cover if anyone from the public get hurt.
I don't think any company would cover us, if random people could join... and if they actually was willing to cover the event, it would be very, very expensive.
And I actually think this is the same in at least parts of the US...