Assign a user to a different site group

1. On the Web site you want to manage, click Site Settings.
2. On the Site Settings page, in the Administration section, click Manage Users.
3. On the Manage Users page, select the box next to the user or cross-site group name you want to change.
4. Click Edit Site Group of Selected Users.
5. In the Site Group Membership area, select the site group to which you want the user or cross-site group to belong.
6. Click OK.

You can assign a user to a site group from the command line by using the userrole operation. The userrole operation takes the url, userlogin, role, and add or delete parameters. Use the following syntax to add a user to the Contributor site group:

stsadm.exe -o userrole –url http://server_name/site_name –userlogin DOMAIN_name\User_name 
-role contributor -add

Note  This does not remove the user from any site groups to which he or she previously belonged.