Question about group permission

From: Ross (Ross_at_nospam.com)
Date: 11/18/04 

Date: Thu, 18 Nov 2004 17:56:22 -0500

Hi there,
I have NIS users in different NIS groups. For example,
user1, primary group group1, secondary group group3
user2, primary group group2, secondary group group3

Now, the user1 created a directory dir1 as this:
drwxr-x--- user1 group1 dir1

On my idea, the user user2 should not have access to dir1 because user2 is
not in group1.
Actually, the user2 has access to dir1.
Is it because both user1 and user2 are at the same secondary group group3?

How to restrict user2 from accessing dir1 without removing it from group3?

Thanks in advance,
Ross