/etc/exports :
/Users -ro

$ sudo launchctl start com.apple.portmap
$ sudo nfsd enable
$ sudo nfsd restart

Check if portmapper is running correctly :
$ rpcinfo -p
program vers proto port
100000 2 tcp 111 portmapper
100000 2 udp 111 portmapper
100003 2 udp 2049 nfs
100003 3 udp 2049 nfs
100003 2 tcp 2049 nfs
100003 3 tcp 2049 nfs
100005 1 udp 812 mountd
100011 1 udp 712 rquotad
100005 3 udp 812 mountd
100005 1 tcp 1014 mountd
100005 3 tcp 1014 mountd
100011 2 udp 712 rquotad

Check if NFS processes are running correctly :
$ nfsd -v status
nfsd service is enabled
nfsd service is loaded
nfsd is running (pid 20126, 8 threads)
lockd service is enabled
lockd service is loaded
lockd is running (pid 9508)
statd.notify service is enabled
statd.notify service is loaded
statd.notify is not running
statd service is loaded
statd is running (pid 9509)
rquotad service is loaded
rquotad is running (pid 20127)

Test if directories are exported :
$ showmount -e
Exports list on localhost:
/Users Everyone