I don't think that there would be a charge, but they would want to know how many people would be dossin' there. My main worry would be that there would be a vast influx of campers and that the site, as well as the 'trowel area', would become packed and unpleasant. I have no wish, or right, to be 'elitist' or to turn folk away, but how can we ensure that numbers are kept to a reasonable level? I rely upon the goodwill of this estate to continue to be allowed to take groups there.
I guess the simplist way is to make it not get too large (which I doubt it would) is to take the estates advise on suitable group size (its not that hugh a site really) then get people to add themselves to the list on a first come first serve basis and confirm nearer the time whether they can make it (there can be a reserve list).
Some meets I have organised on behalf of formal organisations state 'rules' such as being 'known' to someone who knows the organiser, but that doesn't apply here.
I certainly don't know you, and I've only been to one 'Bushcraft' meet orgnised through here. It was fairly laid back, though more could have been specified about toilet (trowel) arrangements and responisiblity for rubbish as not everyone seemed to fully understand their resposibilities.
Toddys advice is sound, make sure they know what sort of group they are getting, and clearly state on the meet thread that people must be responisible for themselves.
Let us know dates as soon as you can, should be a good weekend.