According to the man page, the netgroup only looks at the host, not users for the exports file.
Quote:
netgroups
NIS netgroups may be given as @group. Only the host part of
each netgroup members is consider in checking for membership.
Empty host parts or those containing a single dash (-) are
ignored.
So, if you have NIS set that trusted_group is a group (that user A and B belong to), it won't work. Although the man page also states that the format of /etc/exports is similar to SunOS, I don't recall if you can add a user or group of users in the older version under SunOS.
Change your entry to share it out to specific servers with rw. Then, you may be able to change the permissions and group ownership so that only those two users (belonging to a certain group) can have rw - all others read-only. Not sure if that will work or not. Worth a shot.