I called the phone number listed on the ActBlue page for the Jim Martin TV event and got some more information.
The event is at the Mason Murer Fine Art Gallery,
199 Armour Drive, Atlanta, GA 30324
I asked if there were anything about the event that was graduated per the donation levels on the ActBlue site. It did not seem so, according to the woman at the Martin campaign I spoke with. The "special guest" may be Jim Martin himself, she said - she had no other information.
In any case, I signed on at the "Cast" ($75) level and will attend. I called the Gallery to confirm the date and time and it checks out.
We had planned to donate to Martin again before the runoff and this seemed a fine time and way to do it.