Here you can add or remove users to/from the currently edited role.
If you want to add a user, click the Add users button. The Select users dialog appears. Enter the name of the user or its part and click Search. Now select the listed users using the check-boxes and click OK to add selected users to the role.
More resources on Roles can be found in Developer's Guide -> Development -> Membership, permissions and security -> Security model overview.
Page url: http://devnet.kentico.com/docs/5_5r2/contexthelp/index.html?users_tab.htm