The usage info of "net groupmap memberships" lists "net groupmap memberof <sid>", but that command is invalid. Tested on FreeBSD, Ubuntu and Solaris 11 with 3.5.10 and 3.5.11.

$ net groupmap memberships
Usage:
net groupmap memberof sid
$ net groupmap memberof
Invalid command: net groupmap memberof
Usage:
net groupmap add             Create a new group mapping
net groupmap modify          Update a group mapping
net groupmap delete          Remove a group mapping
net groupmap set             Set group mapping
net groupmap cleanup         Remove foreign group mapping entries
net groupmap addmem          Add a foreign alias member
net groupmap delmem          Delete foreign alias member
net groupmap listmem         List foreign group members
net groupmap memberships     List foreign group memberships
net groupmap list            List current group map