tftv256's description is correct for
Linux (if you wish, you can replace "allowedhost" by "@allowed
netgroup"), but the format for
HP-UX is very different:
/path -access=allowedhost1:allowedhost2,otheroptions
Two useful other options are "ro" (export the directory read-only) and "root=host1:host2:..." which grants root on certain other machines full access to the exported directory, rather than treating root as an anonymous user.