More details to come later, but here's a quick tip:

In the div element that has the ID exhibit-browse-panel, add an attribute called ex:facets whose value is a comma-separated list of Exhibit expressions. For example, this code (in an exhibit showing people)

   <div id="exhibit-browse-panel"
       ex:facets=".age, .work-at, !has-group-member">
   </div>

will add 3 facets in the browse panel: the people's ages, companies (where the people work at), and groups (of which people are members). Note the . and the !. The . operator accesses the named property of the item in question. The ! operator accesses the named relationship pointing to the item in question; so, if the item in question is David Huynh, then !has-group-member evaluated on that item yields Haystack Group, given this data:

   {   label:   "Haystack Group",
       type:    "Group",
       "has-group-member": [ "David Karger", "David Huynh" ]
   }