samba - Jul 2004 - chmod fails if user is in guid but is not uid

Dear samba users,

I'am new to samba and I fail to solve the following problem:

To have access to a share I mount it:

mount -t smbfs -o 
username=john,password=***,uid=501,gid=601,fmask=0777,dmask=0777 
//myserver/myshare /domain/myserver/myshare

If id of user bob is 501, the following commands succeeds:

su bob
chmod 777 /domain/myserver/myshare/myfile

But altough bill is member of group 601 the following commands fail:
su bill
chmod 777 /domain/myserver/myshare/myfile

Is it normal? Is there a way to circomvent that? Am I completely wrong.

Thank in advance for any help.
Regards,
Vivian.