I talked to Tim Kirk about Grey Rock again today. The group I went with does two rides there a year, and the Spring ride will be March or April. Getting "invited" seems to be a matter of someone on the list inviting you to join in, and after a couple of times you can get added to the list directly. Maybe bjeep can comment on this. One of the guys pays the fee in advance for the group and then collects on-site. We can ask if having some GATR rigs participate next Spring, but since the date is apparently chosen only a couple of weeks in advance it would be hard to schedule on the calendar.
Tim says there is a lot of trails for stock Jeeps, but the issue would be finding them for people who don't know the park. None of the regulars with built rigs is likely to volunteer to lead an easy ride. I'm not sure how to address that. Those with F&R lockers and 35+ tires could hang with the regular group.
I also got a clarification on the costs. $1000 up-front to rent for up to 10 rigs at $850 + $150 refundable deposit. More than 10 rigs it's a flat $85/rig, no charge for passengers or camping.