Error While Sharing a Folder with Domain Admin Account

I am using ubuntu and windows server 2008 in a virtual box. I added ubuntu in a windows domain. I have created a user account named user123 during installation of ubuntu. I can share a folder with this user account. After then i logged in as a domain admin.

login: DomainName\administrator, 
password: *****

and try to create a share folder but it shows an error.

net usershare' returned error 255: net usershare: cannot open usershare directory /var/lib/samba/usershares. Error Permission denied You do not have permission to create a usershare. Ask your administrator to grant you permissions to create a share.

I tried to find the solution of this error. and then i thought i should add user account in sambashare group. I used many command to add user in a group, even try to modify smb.conf file but its not working, or maybe my concept is wrong. I don't have a thoroughly knowledge of ubuntu so i don't know how to solve it. Please help me