Yes, IFMEMBER will accomplish what you need.

code:

::If user is in NAV Group, install NAV
IFMEMBER /v "YourDomain\NAV Group"
IF errorlevel 1 GOTO NAVUPD
GOTO MAPDRVS
:NAVUPD
::Install NAV
:MAPDRVS
::Map Drives for all users

That should get you started. I highly recommend you pick up a book or do some reading on creating batch files if you are still confused.