If you're adding a new user, just log into UThink, select your blog from the dashboard, then go to Manage > Add Users. Here's a page that explains the different user roles: http://www.movabletype.org/documentation/administrator/managing-users/roles.html
If you're trying to change a user's roles and all you see is "add" and "delete", you don't have to delete and re-add. Follow these steps:
1. blog.lib.umn.edu > Login > Select your blog > Manage > Users
2.Use "Add users to this blog" to modify an existing user's roles. No need to delete and re-add the user. I know how crazy that sounds. This screenshot also shows that you can check the box next to a user and click "Remove". You can also click the X next to an individual role of an author to remove that role. For example, someone is no longer blog administrator, but you still want them as an author. So click the X next to Blog Administrator rather than using Remove.
3. Enter in a comma-delimited list of x500s, followed by the roles that you want these individuals to have (even if they already have the role of "author", check the box anyway.)