Authenticating a user of a specific group

I am using Active Directiory in Win 2000 Server and Kerboros protocol.
I can authenticate an user whose info is in that Active directory by
Jaas api.
Now i want to check whether a user is in a specific group or not. For
example let i have 2 users 'user1' and 'user2' and a group 'mygroup'
and 'user1' is member of 'mygroup'. In my application if 'user1' wants
to log in then my application will let him logged in but for any other
user (ie, 'user2') my application will show autentication is failed.

how can i do it? i mean, how can i identify that the user is in that
group?

plz help me......

zahid