Is it possible to use Splunk to search all hosts on a domain to identify which hosts have a particular security group listed?
If you are talking about an AD security group, then as a general case, no. That would need to be done using AD tools, unless the AD administrator had chosen to load such data into splunk (an unlikely and bad idea).
you might be able to simulate such a search, if you knew someone who belonged to the AD group (and not many others) and if you know other people who didn't belong to it, then you could find all the hosts that Mr Access had accessed that Ms NoAccess didn't. However, that's very hit-or-miss, since there's no guarantee that Mr Access or Ms NoAccess have accessed everything they could, and there's not guarantee that Ms NoAccess doesn't have another AD group that allows her to get to the locations that that AD group access privilege grants to Mr Access.