Aani,
After reviewing the smb.conf man pages I found some settings that might help you accomplish the behavior you want.
My current share setup in smb.conf is as follows and it allows members of only one group to write to the share, everyone else cannot do so, they will receive a NT_STATUS_MEDIA_WRITE_PROTECTED error.
smb.conf
==============
[test-share]
path = /test-share
public = yes
read only = yes
browseable = yes
write list = +DOMAIN\WRITEGROUP
The above settings worked for me, pretty much what you're doing is creating a read only share for everyone else and only the users in the WRITEGROUP can write to the share.
Here is the output from the man pages for 'write list'
This is a list of users that are given read-write access to a service. If the connecting user is in this list then they will be given write access, no matter what the read only option is set to. The list can include group names using the @group syntax.
Note that if a user is in both the read list and the write list then they will be given write access.
Default: write list =
Example: write list = admin, root,
You can further modify the share to your liking, hope this helps.
Note: This setup worked when I changed the file permissions to my share to 777. Although this is set to 777 the settings in the share due to write list doesn't allow other users to have write permissions.