I have been in some preliminary discussions with provider to bring a couple of her friends to witness my session.  They would not participate except to witness and comment on my humiliation.

Has anyone done that before and how much  does one gift the witnesses.

This is something I've done several times. Everything from taking him out for a night on the town with a couple of girlfriends and showing them that I can get him to do whatever I want to elaborate role plays behind closed doors with up to seven people participating. It certainly ads a special element to a scene to have extra eyes watching!

As far as an appropriate gift for your audience members, I think it just depends on what everyone involved is comfortable with.

Have fun and best wishes!

Not my area of expertise and the rate might depend on the part of the country you are in (more money for big cities), but maybe an additional $100/hour each just for witnessing/commenting?  That would be my best guess.  Whatever you'd pay the Mistress would be substantially more.  Maybe you could just tribute (pay her) a higher fee than normal and She can work out the split with the witnesses/commentators on her own.  Might be good training/practice for subs looking to learn to Dom or Switch.

who really want to see me do a kink/bdsm session....and clients who are exhibitionist.....it is a learning session for my provider friends and total enjoyment for my client.

In our case my provider friends asked very few questions and just observed from across the room. They did not ask for a dime.

Like she said, I would use it as a tutorial to learn how to do bdsm and would love the opportunity!