(imported comment written by SergioBenavides)
To use it in a relevance try the following:
(exists true whose (if true then (exists setting "YOUR GROUP NAME HERE" whose (value of it is "True") of client) else false))
You can get the group name from : HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\BigFix\EnterpriseClient\Settings\Client
As opposed to “member of group of sites” the above relevance can be tested by using the debugger.