Michelle,

You might try printing up a series of stickers which are then given out to
the visitors on the tour, when they pay their admission. They wear them
during the tour, giving the interpreter a visual clue as to who should be on
the tour or not. If someone not wearing the sticker joins in, the interpreter
could notify them of the fact that this is a special tour and the additional
price. If they want to come on the tour, they can purchase their ticket for
"only $xx.xx just in time for the next sheduled tour, from the friendly staff
at the front gate."

You could also take this one step further and give a different sticker to
those visitors who paid for this special tour, and those who did not. This
would especially work if you have multiple sites on the tour, each with a
different tour, or price attached. If there are only the two types of tours,
perhaps the single sticker would be simpler and easier to keep track of.

 I would imagine that these types of stickers (with your museum's logo on it)
would be fairly cheap if done in bulk through some commercial printing place
or regional sign making place.

I hope all of that made sense.

Good luck,

Dan Schoeneberg
Curator of Exhibits
Montgomery County Historical Society
Dayton, Ohio
[log in to unmask]