Did you add the valid users to your smbpasswd ?
Samba maintains it's own user list.
adding user is done by:
smbpasswd -a user_name
Then you will be prompted to enter the password.
Make sure that the users you add, already have
a valid Linux account.
Password shall be the same like in the UNIX account.
Also check for following lines in your smb.conf file:
passwd program = /usr/bin/passwd %u
encrypt passwords = Yes
unix password sync = Yes
REMARK: Perhaps in your distro the passwd program is located
in another directory. In my distro (SuSE) it's under /usr/bin.
If the above lines are not in your smb.conf add them into your smb.conf.
Then restart the smbd and nmbd deamons.